http://www.hourences.com/book/tutorialsindex.htm
Mainly these:
BSP errors - http://www.hourences.com/book/tutorialsbsp.htm
Semisolids - http://www.hourences.com/book/tutorialssemi.htm
Zoning - http://www.hourences.com/book/tutorialszoning.htm
Wow, it explains a lot! I guess I could even make a nice BSP-error-free level like this!

