Recently, I did a deep dive on InfoPath 2007 and Forms Services for a couple of customers. They asked for a number of references that would get them up to speed, so here is what I shared with them. For the most part they are 2007-related but I also included a couple of the "classics" from 2003. If you have one you really like that's not on the list, send it to me.
InfoPath 2007: Designing Form Templates With The New Features Of InfoPath
Feedback from a reader suggested I add this, and I whole-heartedly agree: