I found this old book in my basement(it's from 2004). Would you recommend using it to improve my knowledge or would there be too much deprecated information? I already made a few (rather simple) android apps, but never really got to know java.

it should be ok for basic stuff. the tools are too old.

Their it away. Check the web for "Java ist auch eine Insel". The online version is free and gets regulary updates.
Role of thumb: If it says "J2SE", it's outdated or a trap (like in job adverts, promising the latest tech but looking for J2SE it J2EE knowledge)

@incrholg thanks!

@writeascript HELL NO! Seriously don't touch that! Maybe if it's J2SE 5.0. But there are 4 J2SE versions. If you want a good book for a beginner/intermediate get one of these:

@JavaRules are they expensive? I also think english is my language of choice for learning a language, so thanks for making the first english book suggestion ;)

Edit: I will look them up myself; thanks for your suggestion

@writeascript hey man, so they are really cheap, like 10-15$ on Amazon. If this will be your first language, I really recommend the beginners guide sixth edition, It covers a lot. If you've already programmed before, either just pick up the complete reference, or the OCA then OCP. They have different teaching styles. But honestly I've reviewed a lot of Java books. And 80% of them are bullshit. These ones don't waste your time though! :) good luck!