I imagine something like Lovasz' Combinatorial Problems and Exercises, maybe also in a format amenable to spaced repetition. How many of us are qualified to contribute though? I certainly am not.

We could also write separate exercise books for different topics, and then each of us could specialize and sort of distributed pair-teach-learn in Bittorrent style. For example, Patrick has been helping me a lot (and I think he enjoys teaching me). I'd obviously like to return the favor, perhaps in another subject.