Article: Charles Darwin's Beagle library
Researchers, working over a 30 year period, have reconstructed the library Charles Darwin had access to on his voyage. They have also made the library available on-line, roughly 400 books.
Most of the books were actually the property of the ship's captain.
Most were nonfiction: books about previous sea voyages, natural history books, encyclopedias.
Also included were a KJV Bible and Milton's Paradise Lost (a personal favorite of Darwin's). Darwin also had copies of the New Testament in Greek and German to help him keep fluent in those languages.
Oddly, Thomas Malthus's book, An Essay on the Principle of Population, was not included. This book was foundational for Darwin's theory of evolution by natural selection.