Aha. There is no adequate textbook for the typical [[Mathematical Tripos]] course. So a wikibook rendering is actually quite a sound idea; the main issue not being reliability, but the need to expand exposition.

There won't be one textbook for the entire Tripos, no... you have to get textbooks on each lecture course. Sometimes the textbook won't coincide exactly with the lecture course, but you can usually find one that covers a superset of the course, or two that cover the entire course between them. Lecture courses only coincide exactly with books if the lecturer designs it to do so, which they usually don't (unless they wrote the book, of course).

Also, I expect the courses change slightly each year, as the lecturers doing them change. So writing a wikibook for a specific lecture course at a specific uni really isn't worth it...