As the title says, is there a site that provides (legal) online access to free chess books?
Open Chess Books is a site that aims to “republish classic chess books, with all diagrams transformed in animated boards, and release them for free.”
They only have two books at the moment (Modern Ideas in Chess and Chess Fundamentals), but it's a great resource so far.
Predator at the Chessboard by Ward Farnsworth is a great primer on chess tactics that's available to read for free online or purchase in print form.
The Internet Archive is probably the largest repository of free ebooks. Project Gutenberg has old classics by Staunton, Bird, Ed Lasker and Capablanca.