Jack be Nimble, Jack Be thick, Combat Quarterly, Mining Monthly. buy them all when you're able. Book of Stars is also nice, but I think not as common. Also check the traveling cart Friday and Sunday's, they can sometimes sell skill books. You'll usually need 5,000 gold or more.
If you have the mystery box Qi cut scene, you can potentially get skill books through them mystery boxes, though it's not very reliable since there are a lot of different things you can potentially get.
If you go to the mines and find a infested floor, you can continue to go back to that level of the mines and just spawn kill monsters.
So say that floor 76 has monster invested floor, you can just repeatedly redo that floor all day long for combat.