Ignore the other guy, he doesn't know what he's talking about. MMORPGs are computer games and we're talking desktop games... you know where you interact with people in real life.
Anyway there are 4 Core Rule books. Your original 3 were the Player's Handbook which contains most of the information you need to create and level a character, the Dungeon Masters Guide, which most players don't need to read, but it can be worth your while, and the Monster Manual, which is mostly used by dungeon masters (DMs, or storytellers if you prefer), but which you may use if you summon monsters with spells, have an animal companion or a familiar, or if you use polymorph spells. The fourth book which came out after the first and which some DMs don't allow is the Expanded Psionics Handbook, which adds additional rules for a third type of spellcasting which is a bit more flexible in some regards, and more restrictive in others.
The content of all four books is available at
http://srd.pbemnexus.com/home.html.
This info is not sorted into the four books, but you can pretty much see the demarkations if you are familiar with them. All the psionics are listed under one section, all of the mosters under another, and Most of the DMG is listed sort of under the various chapters of the DMG. If it doesn't look terribly interesting it's probably from the DMG.