I received an email today from a college professor who is interested in starting two new wikibooks on the subject of education. He pointed me to his website, http://wiki-riki.wikispaces.com/ where he has written research papers on the use of Wikibooks to facilitate learning. The one paper, titled simply "Wikibook AERA paper" is a good read and talks a lot about Wikibooks, the WMF, and the various sister projects. Specifically, it talks about the differences between novices to wikibooks, and "wikibookians" who are experienced wikibooks experts. Wikibooks novices, according to the study, appear to be more focused on individual accomplishments, while experts tended to be more driven by the community as a whole. Also, experts tended to be more optimisitic about the wiki development process (better expectations that a book could be "completed", more convinced that wiki was a valuable educational tool, etc).
Another point that is touched on, albeit briefly, was the idea that people new to Wikibooks were uncomfortable in posting incomplete or messy work. The authors surmise that school children are so trained to hand in a "final draft" and receive a grade for it, that they become uncomfortable brainstorming or posting something that isnt perfect online. It's not a bad hypothesis, i think.