In 1975 McLuhan wrote to E.T. Hall:

I have recently done another book, on The Laws of the Media (not yet published), explaining the linguistic character of all human artifacts, hardware or software. (McLuhan to Hall, December 8, 1975)

In the event, the slightly renamed Laws of Media was not published until 1988, more than 12 years later and more than 7 years after McLuhan’s death at the end of 1980. It was left to Eric McLuhan to put together its pieces into a completed volume.

But McLuhan did publish a series of papers on ‘Laws of the Media’ starting in the middle 1970’s:

‘Gesetze der Medien — strukturelle Annäherung‘, Uterrichtswissenschaft, June 1974, 79-84.¹

In an April 3, 1974, letter to Melvin Kranzberg, the editor of Technology and Culture, McLuhan referred to this paper as follows:

I have written various introductions at various times and have thrown them all away. One was called ‘The Laws of the Media: A Structural Approach’, citing Collingwood’s Metaphysics: Where there is no strain, there is no history. Structuralism in linguistics and in aesthetics I am fairly familiar with, especially through the new criticism, and also the history of symbolism.

Eric McLuhan in The Lost Tetrads of Marshall McLuhan (2017) notes that this German paper “included tetrads, in list form, on such topics as Spoken Word, Phonetic Alphabet, Press, Typewriter, Xerox, Telephone, Radio, Electric Media, Money, and Satellite.” (11) Many of these same tetrads appeared in different formats in the following ‘Laws of the Media’ papers.

‘McLuhan’s Laws of the Media‘, Technology and Culture, 16:1, 1975, 74-78.

The continuation of McLuhan’s April 3, 1974, letter to Kranzberg (following the initial section cited above) contained passages which were included, presumably on Kranzberg’s suggestion, in the introductory section of ‘McLuhan’s Laws of the Media’ as it came to be published:

The structuralists, beginning with Ferdinand de Saussure and now Lévi-Strauss, divide the two approaches to the problems of form into diachrony and synchrony. Diachrony is simply the developmental, chronological study of any cultural matter, but synchrony works on the assumption that all aspects of any form are simultaneously present in any part of it. This simultaneous approach is the one I use in ‘Laws of the Media’ herewith enclosed.² However, in the case of any one of them, I can flip to the diachronic approach for filling in historical background and details.
Since electric speeds of information constitute a sort of simultaneous structuring of experience, synchrony, representing all directions at once, is, as it were, acoustic whereas the diachronic, representing one stage at a time, is visual in its analytical pattern. Few people seem to be aware that visual space and order are continuous, connected, homogeneous and static. In these regards, visual space is quite different from any other kind of Space, be it tactile, kinetic, audile or osmic (smell). Visual space alone can be divided.
In formulating the Laws of the Media I have proceeded by induction, even though in the process of induction one discovers many things that could not be merely inducted. The Laws of the Media have been shaped by studying the effects of media so there is always a hidden ground upon which these effects stand, and against which they bounce. That is to say, the Law of a Medium is a figure interplaying with a ground.  As  with a wheel and axle, there must be an interval between the two in order for the play to exist.”³ 

An important observation from ‘McLuhan’s Laws of the Media’:

Although these are called Laws of the Media, only a few of them deal with communications media narrowly conceived. Instead, I am talking about “media” in terms of a larger [ground]⁴ of information and perception which forms our thoughts, structures our experience, and determines our views of the world about us. It is this kind of [vertical] information flow — media — which is responsible for my postulation of a series of insights regarding the impact of certain technological developments.

‘Misunderstanding the Media’s Laws‘, letter to Technology and Culture, 17:2, 1976, 263.

McLuhan’s 1976 letter published in Technology and Culture was originally a 1975 personal letter addressed to Kranzberg. It was a response to a critique of ‘McLuhan’s Laws of the Media’ by Henry Venable, titled ‘Flaws in McLuhan’s Laws’, which appeared in the same 1976 issue of Technology and Culture as McLuhan’s letter,⁵ but was shown to McLuhan by Kranzberg in 1975. The published version of McLuhan’s letter reads as follows:

To THE EDITOR — Mr. Venable’s response to my ‘Laws of the Media’ is typical of difficulties people have with my writings. For example, Venable attempts to relate my “tetrades”, or laws, to logic (the logic of C. S. Peirce). The whole point about my tetrads is that they are analogical. That is, there are no connections between any of them, but there are dynamic ratios. Everything that follows in Venable’s comments is equally remote from anything I have observed or stated. He has not read me at all but has almost at random used my observations as points from which to take off on his own flights. That is, there is no disagreement between Venable and me; there is simply no point of contact whatever.
By the way, I have just come across an invaluable book, Ecological Psychology, by Roger G. Barker (Stanford, Calif., 1968). It was pointed out to me by Edward T. Hall in The Fourth Dimension in Architecture (Santa Fe, 1975), in which he notes: “The most pervasive and important assumption, a cornerstone in the edifice of Western thought, that lies hidden from our consciousness and has to do with man’s relationship to his environment. Quite simply the Western view is that human processes, particularly behavior, are independent of environmental controls and influence” (p. 7).I appear to be the only person who knows why Western man played up these assumptions about his immunity to environmental influence.
Of late, I have been studying more and more the kinds of response to my own work, and there is a uniform response of hostility to any study of psychic effects of technology. Strangely, non-Western man does not take this attitude. The Hindu and the Japanese and the African have welcomed my approach, without demur. As can be seen in my book Take Today: The Executive as Dropout (New York, 1972), I consider that Western man has been going Eastward and inward with the electric revolution. That is, Western man is ceasing to be Western and Eastern man is moving Westward, rapidly, with our technology and hardware.”

Kranzberg deleted the opening paragraph of the August 22, 1975 letter to him for its publication in the journal — McLuhan had indicated that he could make any changes he wanted. Here is that paragraph:

Of course I am interested in Mr. Venable’s response to the ‘Laws of the Media’, if only because it is typical of the difficulties people have with my stuff. For example he says [that my] “starting postulate is that men see the environment as hostile”.  This, of course, is news  to me. He then proceeds to the problem of “non involvement” in relation to books. My entire theme in this regard has been that the printed word creates a habit of “detachment” and “objectivity” on which Western man prides himself. It has been the basis of Western science, at least until the electric age. However, I have always stressed the huge subliminal involvement created by the alphabet, and by the printed word, and have devoted an entire book to this, The Gutenberg Galaxy. By the same token, “objectively” constituted Western man, proud of his detachment, tends to panic in the presence of subliminal response.

The source of this panic may lie in the abysmal border implicated in the possibility of subliminal response. And this may throw light on “the kinds of response to (McLuhan’s) own work”. For if subliminal response represents a deep unknown, the difference between it and ordinary cognition represents an even deeper unknown and one that, unlike a subliminal response, cannot possibly be retrieved. It is this unknown of an unknown, unknown squared, that sparks a “uniform response of hostility”, “panic” and inevitable “misunderstanding”.  

This unrecallable deep unknown is, of course, the transitive medium that is the message.

‘Laws of the Media‘, Etc: a review of general semantics, 34:2, 1977, 173-179.⁶

Some of McLuhan’s observations:

The “Laws of the Media” led me to the awareness that all our artifacts, all our “sensory and motor accessories,” are, in fact, words. All of these things are outerings and utterings of man. (…) Exploration of the “Laws of the Media” opens up the matter of the grammar and syntax of each artifact. (…) All the extensions of man, verbal or non-verbal, hardware or software, are essentially metaphoric in structure, and (…) are in the plenary sense linguistic. (…) The mind of man is structurally inherent in all human artifacts and hypotheses whatever.

All of man’s artifacts, of language, of laws, of ideas and hypotheses, of tools, of clothing and computers ㅡ all of these are extensions of the physical human body. Hans Hass, in The Human Animal [1970, German original 1968], sees this human power to create additional “organs” as “an enormity from the evolutionary standpoint (…) an advance laden with unfathomable consequences.” The “Laws of the Media” are observations on the operation and effects of human artifacts on man and society, since, Hass further notes, a human artifact “is not merely an implement for working upon something, but an extension of our body effected by the artificial addition of organs (…) to which, to a greater or lesser degree, we owe our civilization.”

‘Multi-Media: The Laws of the Media’, The English Journal, 67:8, 1978, 92-94.⁷

Some of McLuhan’s thoughts frpm this paper:

My current writing (…) concerns a study of the laws of the media. (Perhaps it will be called “Phenomenology of the Media“.) This approach to the media is a right hemisphere study which is analogical rather than logical, simultaneous rather than consecutive.

To return to basics in the study of anything whatever means returning to the ground of the central effects of that thing.

The figure-ground approach offers a basic way of studying our culture without getting into controversy about whether a particular thing is good or bad in itself. This study of ground also bypasses the problem of programs and content, whether in books or electric media. The content is not ground but figure, and the overwhelming effect of any technology reaches the public by means of the ground, not by the program or content. 

the extreme point of reversal [is where] any figure, when duly enlarged, tends to flip into ground.