Horizon

Almost certainly the book has over a dozen authors, many of whom may not be explicitly acknowledged. History college textbooks often contain whole sections, perhaps even whole chapters, added by an invisible hand long after the putative author has left the project.

Sure -- my book has about 20 contributing authors.

My real question is whether or not these books are being edited. If so, can I introduce the editor to a couple of concepts that they may find useful, like "clear" or "concise".

Consumers purchase books with prettier illustrations but illustrations increase printing costs and prices.

Yes, but remember that the great majority of consumers are buying these books only because a given instructor requires them, so this is not a normal market. Is it really true that professors demand flashy graphics? That was never my impression, though like many things this probably varies by field.

No doubt textbookese has something to do with the number of authors working on these books, but another reason for the terrible prose has to do with what you might call "institutional style".

Modern textbooks, at least in many fields, are most pointedly not supposed to be the works of a particular person. They are supposed to be -- and they seek to project themselves as being -- neutral expressions of institutional certitude (or, if not certitude, at least consensus). A particular prose style seems to have grown to meet this need, something I think of as borrowing heavily from standard american bureaucratic style, with a healthy dose of technical obfuscation thrown in to discourage quibbling.

I was reading an interesting book by Geoffrey Pullum lately, in which he mentions a study which finds that the likelihood of being successfully published in academic journals correlates very highly with both the reputation of the author and with the difficulty of the prose. I will track that study down someday.

...which again points to the fact that language as a social phenomenon is often just another form of symbolic display, and often only peripherally intended or designed to convey information.

Textbook publishing is in the hands of fewer and fewer conglomerates, corporations that seek high profits. Once upon a time, acquisition editors used to not only acquire a manuscript but also shape it, make sure it was structured to flow sensibly, and send it back to the authors until they got it right. Now they just sign contracts for books that are likely to be highly profitable.

Manuscripts then used to go to a manuscript editor, who'd shape the prose, flip passive sentences where possible, and make lists of points for copyeditors to change. These people seem to have vanished in some
downsizing.

Copyeditors are supposed to check spellings, fix punctuation, apply house style, code for typesetting---straightforward basics like that. Now copyeditors seem to be getting textbook manuscripts straight from the authors' keyboard, barely touched by any intermediary. Sure, we try to Google to check dubious "facts" and make readability improvements, but we don't get the time or the money or, in most cases, the authority to turn a manuscript into a good textbook. And the pay rates seem to be going down.

The other reason I don't copyedit textbooks these days is the profit-making new edition every three to five years. Chapter 4 and chapter 7 are flipped, chapter 9 is split into two chapters, paragraphs of "new studies" are stuck in here and there, and a hundred new references are added. I feel like a crook when I'm complicit in preventing a market in used textbooks, which is the only justification for these frequent updates.

I suspect worse textbooks are on the way. From what I see, textbooks are typeset in India these days, and soon they'll be edited there, too, because Indian staff are cheaper and English-speaking. So you'll have textbooks just like today's but in British English, which will further confuse your students.