I also looked at Lulu, which came in first under the book category. Lulu is basically a site that allow individuals to become self publishers. I can see how teachers might want to use this site to publish books as class projects. I am thinking along the lines of student work, such as class books or poetry compilations.
Biblio is a site that allows users to purchase college textbooks and rare or out of print books. There are times when out of print books are requested by teachers. It would be nice to have a resource like Biblio to purchase an out of print book, if there is nothing else on the market to fill that particular need.
docstoc. is a site that provides worksheets, tests, forms and checklists for educational purposes. While using worksheets is usually frowned upon, there are instances where this site could be very useful. I particularly liked some of the checklists for writing. Checklists always come in handy, in my opinion because through students know exactly what is expected.