Thanks, I'll look that one up.

but don't just read the book. You are very unlikely to grok formal logic without working your way through a large number of problem sets.

I know that very well. I've been filling notepads with tableau proofs for the past few days. I find tableau a lot easier than natural deduction as you can work through them algorythmically, but natural deduction proofs require a strange sort of sideways thinking to them, learning tricks and techniques to take you towards a desired conclusion.