"FrameMaker 5.5.6 for Dummies," by Sarah O'Keefe and IDG books. This has not been updated to FrameMaker 6 but it is close enough to still be useful.
"The Masters Series: FrameMaker 6," by Thomas Neuburger. Check out
http://www.frameusers.com if you want to look into this. Thomas Neuburger authored a popular book for FrameMaker 5, also, but not FrameMaker 5.5.x.
"Adobe FrameMaker 6 Classroom in a Book," by the Adobe Creative Team (yup, Adobe itself). A tutorial-based book. This book is also available for earlier versions of FrameMaker.