Which Book Should I Write Next?

[El Diablo]

David,

"Perhaps I confused the issue by asking "what is best for the world". I really meant what makes most sense for me."

OK, so in algebra terms:
x = what is best for the world
y = what makes most sense for DS
x = y

hahaha

OK, seriously, even with your clarification, absolutely the algebra book. If you think there's a reasonable chance of this book being as good as you think it could be, then for many factors (importance of the book, amount it will help people, satisfaction for you, and money for 2+2) it is definitely the right answer.

Also, I think that book is much more challenging to do well. I bet I could write a "TOP for beginners" or "50 poker hands" book that would be more or less at the same level as your books. Not so with the algebra book (assuming you are correct about your abilities there).