I've found this site to be one of the best sources of data for the game by far and one extremely helpful little aspect of it is that it is linked to from directly within the game. Just hit F10 whilst playing and alist of areas, monsters, characters, quests, items and guilds you have recently encountered will show up.

You're garunteed to find what you need here.

The Official GuildWars Wiki is an excellent source of information compiled by the GuildWars community. There are lots of spoilers, but if you have done it once why puzzle out what you did before and just go get the info. you forgot!