Earlier quoted context omitted.
I see a lot of complaints in amazon reviews about the lack of answers making self-study difficult. It got me wondering...suppose there were a website for autodidacts in math and similar topics? Something where people could post and discuss their answers to exercises. It'd solve the whole problem. Would textbook publishers sue?
I think academics would get pretty annoyed :) Most of my professors just use the exercises from the textbook for homework, usually on the assumption that you can't find the answers online.
Maybe DennisP's idea could do the same thing - only post answers to the odd-numbered questions. Of course, DennisP's scheme would only work for books that actually have decent end-of-section questions, unless people made up extra questions as well ...