Thank you, I know about the wiki but i dont see a single reference to textbook design there.

Text sometimes has a way of sounding severe. For the record, I love typophile!
I will contribute - I just thought maybe one of us may have a suggestion, so that I can jump start my research by piggybacking on my friends experience.

Textbook design has changed (like everything else) in the past couple of decades. College is very different from school, and the lists within disciplines are very different.

One unfortunate trend that developed in the 1990s is custom publishing, which allows a professor to compile content from various sources and get custom books. This segment has grown and the result is that content gets hastily fitted to boilerplate designs. On the other end of the spectrum you have the blockbuster books that tend to look like bad magazine design where every page is different and full of photoshop effects and noise.

Throughout this transition the editorial quality has also shifted. Manuscripts are no longer developed (developmental editors are no more) and the signing editors no longer read them. Design standards have also taken a back seat. As the imprints are sold, repacked and resold, the skilled and better payed workers are replaced with less qualified workers that don't have the same standards.

You also have the churn associated with the education lists as fads in education style come in and go out. Whole Learning? Anyone?

All this is to say, if you are interested in designing books that serve children well you may find yourself with few collaborators.

If I were you I would buy a few used books in the specific area of interest. Look at how they are revised because the industry runs on the forced roll where a new edition is foisted on the students whether they need it or not. Very often they are dressing up old content with very few revisions so the old design/content won't vary much.

I think the industry really needs to rediscover basic design standards. The industry is much more political than you might guess, and large states that represent large adoptions are often given a good deal of control over the content.

Another opinion:
http://www.edwardtufte.com/bboard/q-and-a-fetch-msg?msg_id=0000hB

One unfortunate trend that developed in the 1990s is custom publishing, which allows a professor to compile content from various sources and get custom books.

Unfortunate because the students pay less, or because the book is not five years out of date?

My impression is that the cobbled together books satisfy a vanity of getting to choose what to include, but the end result is not the equal to the texts published on their own.

Speaking from past experience of trying to find a decent textbook to assign for college classes: for the reasons you described and others as well, the textbooks published on their own are often rubbish. The author probably has more knowledge of or interest in some parts of the subject, and might explain those well, but the rest of the book is likely to be an uninspired rehash of what the author found in other textbooks. By picking and choosing, you can get something that is not cohesive, but at least each of the parts is adequate. In a good book the whole should be much more than the sum of the parts, but that isn't happening in either case.