Searching for Books
Google Book Search is characteristically simple to use. For many books, it allows searching of the full text and preview of at least sections of the book.
Project Gutenberg makes available the text of more than 17,000 works that are out of copyright.
The Darwin Digital Library, hosted by the American Museum of Natural History, includes a selected bibliography, mostly historical, but including some recent texts.
Library Catalogs: In addition to public and university library catalogs, many compilations allow searching across multiple libraries; for example, COPAC in the United Kingdom, the Library of Congress gateway page in the United States, or Libweb, a gateway to all kinds of libraries in 146 countries.
Darwin’s works are available online, together with many unpublished manuscripts.
Darwin’s correspondence: Darwin’s letters are being put on line by a major project: Currently, they are available up to 1857 and can be searched in full.
Fisher’s papers and correspondence are available.
Wallace’s papers and correspondence can be accessed online, including the letter (p. 17) in which he proposed natural selection and which was read with Darwin’s in 1858.
Used books: It is now remarkably easy to find out-of-print and used books: for example, Used Book Search, AddAll, alibris, and AbeBooks.