I wrote a 28 page chapter on generics for Core
Java, and I am pretty sure I covered everything the working programmer needs to know.
Thus, I don't think a whole book on generics is worthwhile, although I am sure someone out there will do just that :-)
A great free resource is Angelika Langer's FAQ at
http://www.langer.camelot.de/GenericsFAQ/JavaGenericsFAQ.html Cheers,
Cay