"Introducing Microsoft® Office InfoPath™ 2003" book
Recently, this book was released. It's one of the first ones to cover the additional features for InfoPath Service Pack 1. There are a lot of new features in SP1 (see list of features).
I haven't actually read the book yet but I've heard good things about it from within MS.
Link: https://www.microsoft.com/mspress/books/6511.asp