Witcha lore

>Where should I start on the books?
The Last Wish
The Sword of Destiny
Blood of Elves
Times of Contempt
Baptism of Fire
The Tower of the Swallow
The Lady of the Lake

>Are all the books translated?
The Lady of the lake will be out on March 14 of this year (2017).
There are complete translations of the series into German, French, Russian, Portuguese and Spanish.

If you do not want to wait for the translation, there are already fan translations.
http://forums.cdprojektred.com/threads/16494-Our-Community-Fan-Translations

>Are the books essential for the enjoyment of the games?
No, but the books take place before the events of the games and there are many in-game references to the books. They're a good and cheap read, so it's still recommended to read them. Especially if you enjoyed the games.

>Can I get a quick rundown of the first two games ? Are they required to understand TW3?
While it is recommended to play the games, it's not essential.

Quick rundown :
https://www.youtube.com/watch?v=AXfW2LWK3m4
Not-so-quick rundown :
https://www.youtube.com/watch?v=ncj2xtwyiMo
https://www.youtube.com/watch?v=GqBH33V8KRE

Or, if you don't want to play the games but still want to see what happen in detail, you can watch the "films".
https://www.youtube.com/watch?v=ZHwa-7LBfV4
https://www.youtube.com/watch?v=ieH6PrNIgmI

>Lore series by ShoddyCast :
https://www.youtube.com/watch?v=YrrzVp2HIMk&feature=youtu.be&list=PL7pGJQV-jlzCMqEAP49QApVHNJeZxxdL9

>Maps of the Northern Kingdoms :
http://i.imgur.com/BrKZB20.jpg
https://i.imgur.com/ezypmy2.jpg