Book Announcement (late): "Discrete Mathematics, " by Jean Gallier (Springer, 2011). Chapter 1 gives an introduction to logic using a natural deduction system. http://www.springer.com/mathematics/numbers/book/978-1-4419-8046-5 Amazon (google it) http://www.cis.upenn.edu/~jean/gbooks/discmath.html -- Jean Gallier