When it rains, it pours. I now have two offers, one from my original journal posting (too lazy to dig the url up;)) and another from when I prodded my publisher friend. I sincerely doubt I can do both, or at least a good job on both. But they would both be introduction books, too... I'm leaning towards telling the first folks that I'm no longer interested and focus on working with my publisher friend. But the first folks seem more of a sure thing... I don't know who my co-author is (yet), but he's signed up.
I'll have to think on this one - would appreciate any input.