If by source book you mean setting, then setting wins. Races, cultures, flora and fauna, maps, history...a setting made for and by a RM player would be the cats meow.
A companion that deals with a group, like demons, undead, elementals, feys, ogres, trolls, giants, dragons, whatever, would be nice, as long as it details their culture, history, races, magic, technlogy/weapons/medicine/etc.