I'm not sure how much theory you can actually learn just by reading, or if you could; would it be worth it. Most of the basic stuff is best covered in books. That's your best bet if you want to learn from the ground up. Elementary Harmony by Ottman is a good starting book. He covers rhythm, harmony, melody, and alittle bit of form in a very easy and comprehensable way. I don't know if his books come in German, if not. Check out Paul Hindemith. His books are the best on the subject. Either one of the authors are great, and their books will be extremely helpful. Be prepared to do alittle work though cause both contain several exercises and assignments on each subject as you learn them. Basic theory is something you just need to put into practice to actually learn it.