Classic books on programming

Yesterday I spent too much time checking out the books in 10 Books That Will Substitute a Computer Science Degree. It was discussed on Hacker News. Many of the books come from the ACM's list of classic books. To see a description of the book, click the book's name on the ACM site.

Several of the books are out of print, but some are available freely online. The ACM also offers PDFs of several of the books for purchase, for about $20.