Much of the time, you'll be able to create the graphics in whatever tool you like. Or, you'll do the bulk of the work in one program, then transfer the nearly-finished assets to some other program.

One question you need to ask yourself though is are you going to be animating? If not, then just stick with Photoshop or whatever drawing program works for you. I personally like Photoshop, and do the majority of my game's graphics (which you can see in my sig - that's all in-game graphics) in that.

If you are going to be animating, then that's more complicated. Game animations, broadly speaking, can be pre-rendered or procedurally generated. I prefer pre-rendered, but both have their pros and cons. If you were to end up working on procedurally animated characters, then you might end up working in a program like Toon Boom Harmony, Spine, or Spriter. There, you'd create the individual bits (limbs, clothes, etc.) and rig them for export into the game engine.

If you were making pre-rendered animations, then you probably couldn't do much better than learning Toon Boom Harmony. It's used by Disney, among others. With Harmony, you draw your graphics straight into Harmony using its excellent (and quite natural-feeling) vector tools. Then you rig it and can export as PNG sequences that can be imported into game engines

If you want to learn Toon Boom Harmony, then I highly recommend the digitaltutors range of tutorials: http://www.digitaltutors.com/software/Toon-Boom-Harmony-tutorials

Are there any books or tutorials for 2D artists?

Sure, there is a mountain of them. I have about 20 Gb on my PC. Barely used ten of books so far) However, there are far less books which helps you learn how to draw specially for games.

You should try to read this old thread. http://www.gamedev.net/page/resources/_/creative/visual-arts/the-total-beginner%E2%80%99s-guide-to-better-2d-game-art-r2959. It's a step by step tutorials for beginners. Pretty old thread but pretty good tutorials.