Catalog for 43 national libraries of Europe, contains some digital collections of books that are out of copyright.
Hathi Trust Digital Library
An online book resource with some full text books (out of copyright) and searchable books (with snippets and page numbers).
Brill Books and Journals Online
Brill is an academic publisher with specialties in humanities and social sciences. Search with your keyword at the left under Refine Search > All. A green icon next to a title will indicate full text access.
The Online Books Page
A listing of full text books online produced by a digital library planner and researcher at the University of Pennsylvania.