Seally: Book: Specters, Wraiths, and the Damned (for Wild Hunt entry)
Quests: Dead Hand of the Past (Chapter I), Hunting the Wild Hunt (Chapter IV), and complete the game (for King of the Wild Hunt entry)

Since GOG.com apparently doesn't trust me yet, I can't post links, but I can say that the Gamepedia Witcher Wiki and the Witcher Wikia (see notes) has almost everything you'd want to know about the game (including some technical issues scattered in relevant articles) and also some interesting tidbits from the books. For the first game (the info for the Witcher 2 seems less comprehensive) there are also useful info like Library Guides, Shopping Lists, Grinding places and so on. Listed for your convenience of course, in case you don't know what to search for.

NOTE: In case you're wondering, yes, there are both an official Wiki (Gamepedia) and an official Wikia. I wasn't present when the split happened, but it had something to do with Wikia's censorship policy, advertising policy, and their tendency to do things without asking the admins, which soon escalated to admins and main contributors being locked out. You can still search for the thread discussing this through Google if you're curious, which is definitely more trustworthy than my retelling of it. Wikia's better at Google PageRank manipulation though, so it usually shows up first, and probably has more viewers. Content-wise they are extremely similar, though I personally edit only the Wiki, not Wikia. For me, Gamepedia doesn't annoy me with ads and the background looks better.