GNU Chess is open source and good enough that a serious adult player I know speaks highly of it (on the hardest setting - DS8 plays it on the easiest setting and occasionally beats it to his great delight). He likes the books by Murray Chandler; had Chess for Children very early and now reads How to beat your dad at chess (although what a terrible title; DS beat his dad long before he beat his mum ;-) fairly often. The next one is
Chess Tactics for Kids, which we don't have yet, but which looks as though it might be good for someone a bit more serious than DS is yes, covering the kind of thing mithawk's talking about.
