I've always had an interest in the history of science, particularly of evolutionary biology, and enjoy reading classic works in the field. While I'm lucky that I live near the Library of Congress, with its fabled collection, it's nice to be able to read from the comfort of my own home (the LoC is non-lending). What to do? One possibility is to read online books. Everybody probably knows that the works of Darwin, Huxley, and other Victorians are available on the net, but you may not know that even early 20th century classics are now available, thanks to the digitizing efforts that have been going on in various university libraries. Some may dislike the idea of reading on-screen, but there are services like CafePress and Lulu that will print and bind a PDF file for you for a small fee (and eventually, we will have practical e-ink devices that will make printing unnecessary. Oh, and flying cars and personal robots too).
Anyway, here are a few significant books that I've discovered on-line:
Hugo de Vries' Species and Varieties, Their Origin by Mutation (1905) -- The classic "saltationist" work by one of the rediscoverers of Mendel's laws, this is a good example of early thought by geneticists on the issue of evolution -- de Vries considered mutation, rather than selection to be the primary force behind evolution.
Thomas Hunt Morgan's The Physical Basis of Heredity (1919) -- The book that established the gene as a real entity on a chromosome rather than an ideal "factor".
R.A, Fisher's The Genetical Theory of Natural Selection (1930) -- Considered the most important advance in evolutionary theory since Darwin, this book ended the "eclipse of Darwinism" caused by people like de Vries by explaining selection in terms of Mendelian genetics.