There is only one book so far for DBPro, you can get it from Amazon. Published by Premier Press in their Game Development Series its called -
Beginner's Guide to DarkBASIC Game Programming.
I have the book and it is aimed (obviously) at a complete noob, but it is well written and quite humorous in parts. It will take you from not knowing what a variable is up to a reasonably advanced level. The final game is an online multiplayer car game, so thats fairly advanced!
In addition it has a cd with it that has a .pdf file with the whole set of language commands and their syntax - very useful.
By the way, the book covers both DBv1 and DBPro.
It's never as easy as it looks!