What exactly are mastery books? And which ones are there for Dark Knight and are there any for third job? Also where should I train at level 50?
Mastery books are only for fourth job skills. The drop list can be found here http://royals.ms/forum/showthread.php?t=792.
Dan covered the Mastery Book part, but as for training I'd suggest doing whatever you want (maybe Straw Target Dummies in Mu Lung or something) until you're around 55 or have enough DEX to consistently hit the monsters at Ghost Ship 1 and 2 in that one fancy metropolis town that you get to from Kerning lol. Train there till you can go to Wolf Spiders in NLC (around level 80). Stay there until you can go to Himes in Mushroom Shrine (95-100ish) and go there until you can go to Skeles in Leafre!
