OreillyAndAssociates have made many of their out-of-print books available at http://www.oreilly.com/openbook/
Among the books available are TheCathedralAndTheBazaar by EricRaymond; FreeAsInFreedom, a new biography of RichardStallman; and Web Client Programming with Perl.
There are also books on DocBook (DocBookTheDefinitiveGuide), LinuxOs, and many of their out-of-print books.
See: BookList
CategoryBooks, CategoryOnlineBooks