The Hacker Shelf is nice crowd-sourced guide to (legally) free books on various computational and mathematical subjects. The topics page gives you an idea of the breadth of material available.
posted by philipy on Mar 15, 2012 - 24 comments

Richard P. Feynman { Information Junkie → PhD → Atomic Bomber → Professor/Lecturer on Physics + Mathematical Artist [DIY] + Nanotech Knowledgist → 33.3% Nobel laureate + QEDynamic Speaker + Tiny Machinist + Challenger of Conclusions + Best-Selling Writer –X– Busted [outside Tuva] → Star Trek TNG Shuttlecraft ↓ Pepsi Black/Blue ↑ U.S. Postage Stamp } ∞
posted by Poolio on Sep 16, 2007 - 51 comments

Autodidactic goodies on a budget: Free computer books and online lectures, seminars and instructional materials from a variety of renowned institutions.
posted by Blazecock Pileon on Nov 21, 2006 - 19 comments

Charles Babbage's Difference Engines. One built in 1853. A subsequent design completed in 1991. And again in Lego. Both designs recreated in Meccano parts. [more inside]
posted by slimepuppy on Apr 26, 2006 - 11 comments