Start with tutorials. I also learned a lot from getting my hands on .FLA files and looking how other people made there flash content. It really helps to see how to organize the layers.
I started some tutorials... but can't really master it with those tutorials! do you recommend a book?
Books help to a certain point. You should search for .FLA files on a search engine, and open those and see how it works and such. Flash get more complex when you start making more details and having multiple actions on a single clip. So I suggest just start working on basic things than slowly building up until it is what you want.
lynda.com is good as was said earlier. Search for walkthroughs, or look at what others have done. Watching lots of good flash helps. Also, go to places with lots of submitted stuff too. Maybe of them have submitted tutorials, and it may help to see things explained in more than one way. When I tried in in college, I found newgrounds.com helpful for that.
Just play around. Find a tutorial that interests you and do what it takes to complete it with 100% accuracy. Then... do another one.
http://2advanced.com/ thats mastering it but yes like the other said get a hold of .fla's, keep following tutorials and work your way up.