What's next, I don't know yet, but I definitely don't feel like writing another one. I heard that this feeling changes after 6 months or so, is that true? ;)
You remember the two things you need to do to write a fine programming book?
1 - pick the correct topics
2 - teach them well.
I know how hard it was in this book of yours to pick the correct topics. Harder than usual I'd say. That was just the nature of writing an GP book. So now you know how to teach well, if you get an idea for another book where the topics to teach are more obvious, I'd guess it would be a lot easier to write a second book ;)
I'm looking forward to getting a copy of your book!