I'm working right now on the human biology section of a high school biology textbook for a major publsiher. I've done chapters in nursing textbooks and early childhood ed textbooks for McGraw Hill/Delmar. Amazingly, I got started doing this by answering a blind ad in the New York Times. It was just luck that I had an ideal background to write textbook features (the stuff in boxes that lighten the text) for an early childhood ed preschool health and safety book. (I had an MS in physiology and was teaching at a training school for nannies) and the book packager that was doing the book was only a few miles from my home. Their other writers were spread across the country, and I think they liked the idea that I was someone who could actually drop in at their office. Once they discovered I could write and had a science degree, they gave me work on some of their nursing textbooks and finally a whole book. This answer is probably not helpful, because it would be hard to replicate my entry into textbook writing, but there does seem to be a scarcity of people in the sciences who can write simply and clearly for textbooks.
What you do need to know is that the author of a textbook and the writer of a textbook are usually two different people. My experience has been that the textbook publisher researches the various state standards (this is for lower than college level) and gets a bunch of content experts to put together an outline that covers the topic and meets the state standards in the larger states (CA, NY, TX). Then they hire a writer to work from the outline and actually write the text. The outline not only tells you what to cover, but how many pages should be devoted to each section. Usually you can re-arrange things in the outline to some extent, but meeting the length is critical. The trick is knowing what to leave out, and that is harder than it sounds. The other critical factor is the ability to simplify and hit the right reading level for the target audience (you're given a sample of what they want as a guide). Sometimes I have been given research material and/or competitors books to help with the project. Sometimes I do all the reseach on my own. The writer can suggest graphics, but doesn't have to find them. Usually someone else does the sidebars and features and a professional indexer does the indexing. Basically all the writer is responsible for is the running text and usually some review questions for each section or chapter, but none of the bells and whistles that make the book visually appealing. The writer is also responsible for producing a list of works consulted or used in research for each chapter or section of a chapter. Once the material is written, it gets edited by an in-house editor, the expert(s) whose name is going on the book as author reviews it for accuracy, and the publisher or textbook packager does all the rest - puts together the graphics, gets permissions to use photos/art, copyedits, proofreads, indexes, and sells the book. Contracts are work for hire. The writer usually only gets a tiny credit somewhere in the front or back of the book. Usually multiple writers work on a a book doing different chapters or sections and in-house editors smooth them out so that they don't sound too different in style from each other. Payment, in my experience, varies from about 50 cents to one dollar per word, and I've never been asked to do revisions of my submissions.
If you are interested in this area, you might want to join AMWA - American Medical Writers Association. You can also get your foot in the door by figuring out who publishes encyclopedias that cover your field and trying to get work writing articles on your specialty (virtually all encyclopedias that i know of are done by freelance experts/writers).
Hope this helps you figure out if you want to be the author of a textbook or the writer. If you have more questions, I'll try to answer them. I don't know many other textbook writers, so I don't know how universal my experience is, although I have done this for several different major publishers, and the process has always been similar, regardless of the publisher.