Well the first game was full of characters and references to events from the books, and it kind of assumed anyone playing knew what was going on. You were still able to understand the story though, so this one shouldn't be any different.
There will be a lot of returning characters and you can import your end-game save from the first one over to this one thus allowing your choices to carry over and be reflected. It's very likely that you'll be able to understand the story of TW2 perfectly fine without playing the first, but TW1 was a fantastic game and this one won't be out until sometime next year so I'd suggest playing through the Witcher. --- Do not lend or make illegal copies of this post.
Even though I haven't read the books, I didn't have any problems following the first game, so I'll assume that they'll make it easy for someone to follow this game even if they haven't played the first.
It would be unwise not to play the first Witcher. That is a great game and a great experience. It's cheap now so it's better to play that game than waste money on new games that are coming out because it is better. So, just buy the first Witcher, enjoy it, and then play the second one when it comes out. You won't regret it, unless you don't like the gameplay...but that's why you need to do research before anyway :P.
As everyone else said, The Witcher is a great game and definitely worth picking up. While you'll probably be able to understand the story if you only play TW2, you'll gain more from the experience if you play TW1. You can also import your save. TW1 is also fairly cheap now, so I would pick it up. --- PSN: Phazor58 (Put gamefaqs/gamespot in the message or I will not add you) Steam: [YEAH]Phazor (Julius Caesar avatar)