Let me first make a statement:
The main purpose of Go Books is not to help you improve but to sell copies.
Of course supporting go professionals and their intermediates with your money may be a just cause for the whole of the go world and I have no intention to warn against it. However, if you want to improve, books are auxiliary at best, hardly necessary and detrimental in some cases. Many people (including me) expect reading books to make them improve in a magical way. Books are a, but not the only, source of new ideas and ideas must be put into practice. Playing, reviewing and doing tsumego are much more important.
Read Benjamin Teuber's guide to becoming strong if you want to improve.
So, from a pedagogical standpoint, there are only a few books I recommend:
As said, I would very much advise against joseki books. Professional games will anyhow contain the same ideas, without forcing you into set patterns. I'm guilty as anyone in attempting to classify opening patterns here at SL, but when taking a step back, I can see this is mostly helping our encyclopedia rather than my go.
However, serious gaps remain in Western go literature. Especially basic technique is not explained, probably because the original authors reside in a culture where basic technique is acquired naturally and there is no demand for books about it. Minue has understood this very well and wrote a haengma tutorial for beginners series here at SL, which not only explains basic technique but also talks about the fundamentals of the game. As such, it does a much better job than the universally hailed lessons in the fundamentals of go which is a fun read but only meekly provides you with actual fundamental understanding of the game.
So, read Haengma tutorial for beginners.
There is of course another value in reading books than the mere pedagogical one. Like a teacher, a book should not only teach you something, but also inspire you to continue your path to master the game. In other words, not only the how but also the why. Being a very personal matter, here are the books that inspired me to stick with this fantastic game, as they were fantastic books.
I have left out Invincible, probably because I wanted to keep the list to 5 items, but one cannot really omit the greatest endeavour in Western go literature from any serious overview. I think my feelings towards it are more of respect and awe than inspiration. Its sheer volume can put off its reading at times and anyhow a thorough review of a pro game is never an easy read. I'm afraid Invincible is somewhat equivalent to Ulysses: a must have but never read.
There are many more go books which certainly deserve their spot in the pantheon of go literature but I have deliberately kept this list to the absolute minimum, since most players are looking for ways to improve, which is also what go books implicitly promise, but alas go books are only a minor element in an efficient improvement path.
"The gap": Yes, there are still big gaps in Western literature, but "basic technique is not explained" is a wrong description of the current situation. We are not in the situation "There is the Kageyama, it explains too little, and there is nothing else!" any more. In particular, there are First Fundamentals (which covers all principles a double digit kyu needs to become single digit kyu, except that a player might need more reading problem books, which exist e.g. as Graded Go Problems For Beginners), Joseki Volume 1 Fundamentals (which explains most of the important move / stone types and meanings) and similar books. There are also books that explain relatively few basic techniques, but few is more than "not explained": e.g., Wilcox's book explains sector lines. There is, in the current Western literature, a still major related gap about what I call (basic) techniques: short standard sequences to deal with specific shapes (such as cutting the keima). However, by your reference to the Kageyama, you show that your use of "techniques" is not so specialised - instead, you also imply reference to all sorts of fundamentals. Surely there can be more about basic techniques in Western (or worldwide) literature, but the related major gap is not any more for basic techniques. Now, I see the major related gap in the single kyu and dan ranges. Your statement "basic technique is not explained" not only needs correction, but for a softer version you should also explain with more careful justification which basics you think that are missing. Your current description is just outdated by already better available literature.
Dieter: I must admit I have not read your books and am willing to accept they fill gaps.
"main purpose": You make the statement "The main purpose of Go Books is not to help you improve but to sell copies.". Are you serious? Book authors may or may not have serious commercial interests. If they have, then for themselves, selling copies is the or one of the major purposes indeed. However, not everybody is a book author. The readers have different objectives, such as wishing to improve in strength by learning from books. Go players are very different: some do not read any books (if we may believe rumours) - some read books, and some of those improve very quickly due to learning from books. E.g., I started surpassing DDK level and reached 3d very quickly mainly because of learning from books. Whether books are detrimental or necessary depends on the player, doesn't it? However, it depends also on which books a player chooses (not) to read. Of course, other means of improvement are also important: playing, reviewing, teacher input, problem solving (which can be done from books, would you believe...) etc. You say "Playing, reviewing and doing tsumego are much more important." - I say "Good books are very important, but the other means are also important". Since you express doubts: why? Have you read the wrong books? Have you read them in wrong manners? Have you read too few books?
Dieter: I also believe you are an exceptional case of an author that does not seek a means of sustaining the cost of life through writing go books but as a genuine pursuit of truth. However, once the investment is done, I'm sure you seek to sell copies to get at least the investment back.
Robert Jasiek: I write books for several purposes, among which 1) earning money (or you might say: avoid starving), 2) researching in go theory as a, to use your words, genuine pursuit of truth and 3) spreading go theory to all players are about equally important.
"advice against joseki books": You say: "I would very much advise against joseki books." Why? There are dull (dictionary only etc.), but there are also valuable (with general go theory explained carefully) joseki books. Even from the dull joseki books, one can learn a lot and improve much if one invests very much time and knows how to study well. In the early 1990s, there were only dull joseki books, so I had no choice but to invest much time and much thought about how to learn from dull sources. Result: ca. 3 ranks improvement. When you say "this is mostly helping our encyclopedia rather than my go", then you have not chosen valuable josekis books and have not figured out how to learn well from encyclopeadic joseki books.
Dieter: No doubt a lot of knowledge can be acquired through a proper study of joseki. However I doubt the efficiency of such study and moreover I am afraid any alternative joseki book only adds to the problem of overemphasizing the opening and its potential set patterns. I'm convinced one does not need to acquire all that much knowledge in order to become 5d. Play, review, do problems and get a source of ideas, which can be a book but also pro games. Beyond 5d few people will get: only those who are talented, start young and prosper in an appropriate study environment. None of these requirements is fulfilled by a book. Therefore I'm overall skeptic of the improvement potential of books. Perhaps the best way to improve through a book is writing one. And again, that's a very inefficient way.
Robert Jasiek: Studying joseki for the sake of set patterns / set sequences is, of course, fundamentally wrong. My joseki books (and my joseki learning study when I was a kyu player) follow an almost contrary objective: to overcome set patterns / sequences by understanding every related aspect of go theory in general so that it becomes applicable not only to josekis but to preferably all aspects of positions. - If you will have become 5d with only little knowledge, please report:) However, you would not believe just how much every 5d+ knows... Despite my great efforts of generalisations, the amount of knowledge simply is great. Yes, one does not need all the knowledge of an arbitrarily considered particular 5d; I have unlearnt too useless knowledge. What was a dog shape? It was immaterial. - A single book cannot fulfil all the requirements you mention: it does not play for a player:) Players acquiring knowledge must also learn to apply it while playing. - Improving by writing books? This luxury problem exists only for 5d+ without teachers fitting the student's thinking style (as in my case) and without regular access to equally strong players (not my case).
Oh, I believe already am unbalanced in terms of acquired knowledge compared to playing strength. Obviously a lot of that knowledge is useless (that's what makes it skewed). Playing games mindlessly will also make for a skewed balance between time invested in playing and actual strength but if one includes regular and sharp review I'm quite confident one can get there with a minimal reading of books. I cannot confirm from actual experience: I stalled at 2d but surely not due to reading not enough books. I just couldn't invest the mental energy in playing and reviewing anymore. The energy required to stroll around a library is much lower #;-7.