Free (Creative Commons) Book on How To Do Proofs
people.vcu.edu
people.vcu.edu
To see a mathematics textbook from VCU appear on Hacker News was a welcome sight indeed!
Though, it's still a great art school.
Example: in section 6.4, it is easy to prove that 6a^2-2a+7 is even iff a is odd, and the direct proof gives a better intuition that the proof by contradiction and contrapositive proofs described.