The next step after IFL?

I have added a page of suggestions of what to read after my intro. logic book. I'd be particularly interested to hear comments, take suggestions, etc. And it isn't just a question of putting a favourite sequence of books in front of people for them to battle through, so that they can eventually jump examination hoops. There is also the matter of inducing a certain amount of that elusive desired quality we call 'mathematical maturity'. How do we best help bring that about?