Here are a three specialized databases that do a good job providing info about and/or access to thousands of digitized books and texts.
+ Digital Book Index
This database (free to use) contains information about more than 100,000 digitized books and texts from over 1800 publishers. About one third of the titles are available in full-text format at no charge while other titles are fee-based. Registration, also free, is required to access the database. You will need to give an email address but can make the choice to opt out of any future mailings. The database can be searched by author and title. You can also browse by author, title, subject, and publisher.
+ The Online Books Page
I wrote about the OBP for SearchDay last year. The database currently provides searchable access to over 20,000 free full text books. The OBP comes from the University of Pennsylvania.
Search for commercially available ebooks from more than 400 publishers. Listings come from Overdrive's Content Reserve Digital Content Marketplace. Search by: