Because it gets into some of the challenges and design thoughts behind the process, I like -
Game Design: How to Create Video and Tabletop Games, Start to Finish by Lewis Pulsipher
I haven't read it, but I've read good references for -
Kobold Guide to Board Game Design
A smaller community site: bgdf.com
(board game designer forums) has a great backlog of discussions on the topic.
Also, boardgamegeek.com has a designer forum, but it has poor signal to noise.
But some of the best material out there is in podcasts...
http://boardgameuniversity.libsyn.com/
