I love Sally Swift's Centered Riding. It's more of a feel and imagery focused sort of book, which, if your boyfriend is more mechanically-minded in terms of understanding and conceptual comprehension (most guys I know are), may or may not be up his alley.
Also, you might take a look at Lendon Grey's book. More systematically written, progressive exercises that are excellent for both a learning horse and a learning rider.
If your boyfriend is really interested in diving headfirst down the rabbit hole after he gets his feet wet, I highly recommend this book:
http://www.amazon.com/Complete-Training-Principles-Classical-Horsemanship/dp/0879802359/ref=sr_1_1?s=books&ie=UTF8&qid=1325032869&sr=1-1It's a bit dense at times, but the author is a true master and well worth reading. Not really a beginner's book, but If you really want to study the methods of a master, this is the book.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown