This same question was recently asked on another forum. I am a former user of FrameMaker and have been getting reacquainted after some time away from it. I found that the learning curve on FrameMaker really isn't any harder than most of the other software packages that I have used. They all have their little idiosyncrasies that you have to adjust to.
Since version 9 was just released (in the last couple of months), I don't know if you will find any non-adobe created books on the latest release. However, I would think books on versions 7 and 8 would give you a good foundation.
I saw a couple of people mention the following:
O'Keefe and Loring, "FrameMaker 7: The Complete Reference",
ISBN 0-07-222361-8, published by Osborne/McGraw Hill, which has now
been republished as "Publishing Fundamentals: FrameMaker 7", ISBN
0970473338, at: http://www.scriptorium.com/books/fm7/index.html
They also have an excellent book on Frame 8, "Publishing Fundamentals: Unstructured FrameMaker 8", ISBN 9780970473349, at: http://www.scriptorium.com/books/fm8/unstrfm8.html
In addition, they offer their workbooks for free on a wiki: http://wiki.scriptorium.com/tiki-index.php
Another user suggested Classroom in a Book, Framemaker 7.0.
I personally haven't used any of these, but you might give them a look.