Internet Archive Texts - "The Internet Archive Text Archive contains a wide range of fiction, popular books, children's books, historical texts and academic books"
Open Library - "an interface to the published books on archive.org and links to other online book collections"
HathiTrust - contains over 10 million digitised volumes, including many (all?) of Google Book's and the Internet Archive's available full texts, but also a lot of volumes related to history and genealogy from other sources.
Google Books Library Project- "An enhanced card catalog of the world's books . . If the book is out of copyright, you'll be able to view and download the entire book".
The British Library's Catalogue Search allows you to search for over 13 million books in their collection.
Bibliography of British and Irish History - "The Bibliography of British and Irish History (BBIH) replaced the RHS Bibliography on 1 January 2010. It contains all of the data previously available in the RHS Bibliography, including its partner projects, London's Past Online and (up to the end of 2009) Irish History Online. It covers not only British and Irish domestic history, but also the history of British and Irish relations with the rest of the world, including the British empire and the Commonwealth. . .Subscriptions are available to institutions and individuals."
The Clique - Booktrade and Library Information Services. "Booktrade information for British booksellers, librarians, learned, literary and historical societies, associations and their members for over a century." Information relating to old, rare and out-of-print books in Britain and lists of societies, booksellers and libraries etc.
The British Book Trade Index, or BBTI, is an index of the names and brief biographical details and trade details of people who worked in the book trade in England and Wales and who were trading by 1851.
The Archive CD Books Project - "The Archive CD Books Project began in the UK in March 2000 making reproductions of old books, documents and maps available on CD to genealogists and historians while co-operating with libraries, museums and record offices by providing money to renovate items in their collections and by donating books to their collections."