Creatures & Treasures is the RM2/RMC book that combines things you kill with things you take from the things you kill: very old school organization, there. RM2 had two additional volumes, as Chorpa, noted, also called Creatures & Treasures, but with a II and then a III added on.
Creatures & Monsters combines the C&T monsters (all of them, I think) with most of the C&T II monsters (the only thing I've noticed missing are Entities from Deep Space - I think because there were issues from them having originated in Dark Space, a different product, but I'm not sure about that... all I know for sure is, they make ideal Cthulhu Mythos substitutes for Rolemaster, so it's a shame they got dropped) and monsters from some other sources (a few monsters scattered here and there in the regular Rolemaster Companions of RM2). Entries are also somewhat expanded compared to C&T and also some adjustments were made in the transition to the RMSS rules. I don't think there are any C&T III monsters in C&M.
RMSS dealt with magic items separately, in Treasure Companion. RM2 had a roughly corresponding book, Alchemy Companion, but while there is some overlap, I am very glad to have both. If Alchemy Companion is available as a pdf, it is one of the RM2 books it would be worth having as a RMSS/FRP GM.