Free Springer eBooks (PDF) Older than 10 Years - 50k+ Titles
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.
Also see the recent deal for discounts on newer titles that expires this week.