Mutants in Avalon was a very nice book.
In addition to insect monsters and new animal species, it included the Druid background allowing for magic to be brought into the system, it was a fudal/kingdom setting so it implimented a system for running a class society, and included Arthurian NPC's (Merlin, Morgana, Arthur, The Knights if the Round Table, etc..). It also gave some infromation on a European human army SAECSN (updated in the AtB 2nd Ed. Book, Page 166).
The art in Avalon was awsome, especially the
cover
The only thing I didn't care for in Avalon was - oddly enough - the Arthurian angle, but I figured that was optional.