Here is an improved version of the five chapters on propositional natural deduction for IFL2. (Exercises to be added, which will fill some gaps, like noting the equivalence of DN and Classical Reductio, given the other rules, or dealing with biconditionals.)
As I have said before, one advantage of basing an intro logic book on trees (as in IFL1) is that people don’t get very exercised about how a tree system should be developed. By contrast, people get decidedly heated about the best form of natural deduction to adopt. And I have changed my mind more than once about how to do things (having explained things rather differently in classes over the years). I have ended up with a more conservative Fitch-style system than in previous drafts. But that is still probably not going to satisfy half of those who urged me to go for natural deduction in the second edition: it doesn’t even fully satisfy me. But there just is no meeting all the desiderata!
As always, all comments (other than variants on “you have written the wrong book!”) and all corrections will of course be very gratefully received.