I think it comes from the fragmentation of a subject which follows from the project of having to say something about every aspect of it, and all at about the same academic level. That is of course inimical to privileging any aspect, and imposes a spurious even-handedness on the whole, which is particularly exacerbated when awarding and accrediting bodies start making demands, such as in the case of requirements of some states in the US to deal with evolution and "creation science" evenhandedly. *
Whatever the reason, the focus is usually on isolated topics divided by sub-headings, with copious use of box-outs, and exercises and other sign-posting devices such as "in this chapter you will learn about..." or even specifying by number the competences or performance criteria addressed. It becomes impossible to read for any length of time to get any semblance of a coherent argument.
This turns the reader into more of a raider, who turns to the text-book in search of a particular gobbet of information, and makes her get-away as soon as she has found it, preferably in a form which she can use as a gratuitous quotation in the assessment she is working on--after all, if there were no assessment in the offing she would not be touching the book in the first place.
I am becoming more convinced that text-books are inimical to deep learning. That's one reason why I am working (together with a friend and former colleague) on an alternative or even antidote to them in the teacher education field. This will--we hope--be far from even-handed. It will be about, appropriating Nietzsche's phrase, "philosophising with a hammer"**.
But I'm beginning to understand the text-book authors' problem. How do you assemble the content so as to constitute a coherent whole? Actually, I was well aware of this years ago—that was when I latched onto the idea of a hyper-linked "site" which would enable users to link pages to each other according to their own interests and concerns rather than a pre-determined pattern. In a sense, I am taking a step backwards in thinking in terms of a book.
I suspect that what lies behind this is the epistemology of writing about a professional practice. That is epistemologically unstable. It all too easily flips from a commentary on practice, to a prescription of criteria to which practice ought to conform—but it is much more difficult to go the other way.
And that is what has happened to the text-books—on teaching, at least. So they have helped to redefine the trope in terms of playing an academic game, which is about passing assessments which may well be far removed from valid measures of what really counts. No wonder all the students do is raid them for ammunition in the assessment war.
I'll let you know if I find a way out of this jungle. (Apart of course from the obvious one—don't use text-books...)
* And of course do read Feynman's expose of maths textbooks from 1964, which brings in a whole different dimension of corruption in the US system—as well as being hugely entertaining as he always was.
** That is the sub-title to his Twilight of the Idols ("Götzendämmerung", 1889), about which Walter Kaufmann comments:
"It is usually assumed that he means a sledge-hammer. The preface, however, from which the image is derived as an after-thought, explains: idols 'are here touched with a hammer as with a tuning-fork'." (1954: 464)Kaufmann, W (ed. and tr.) (1954) The Portable Nietzsche New York; Viking
This post links—in that it follows from the same stimulus of planning our book—to another On an Academic Conscience coming shortly.