The top three most important card books are The Royal Road to Card Magic, Expert at the Card Table, and Expert Card Technique, which I suggest should be studied in that order.
Then, I'd go for some classics like Dai Vernon's Inner Card Trilogy and Ultimate Secrets of Card Magic, and maybe some other Vernon stuff like Dai Vernon's Book of Magic, Select Secrets and Early Vernon. They've also re-released the Vernon Chronicles series, so I'd pick those up afterwards, as they've been out of print for a while so there'll be some stuff in there that not many people are familiar with.
If you want a quick fix of a huge amount of card material, then I'd go for Marlo's Revolutionary Card Technique and Cardially Yours. Those two together will keep you going for years.