Hacker News has mentioned this site a few times in recent days…
A search for books prior to 2006 returns 56,245 titles, and a further 18,977 titles that are limited to a preview.
No registration, just browse titles and download in PDF. I have no idea if this opportunity expires.
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Alan Turing: Life and Legacy of a Great Thinker
http://link.springer.com/book/10.1007/978-3-662-05642-4