How bizarre! I'm also learning German and had a, "am I on the right website?!" double take moment when I came onto this topic!
I have Pünktchen und Anton on my kindle - think you can get most of Erich Kästner's books on there. I also have the audio book on my phone for a bit of listening practice.
Although not German originals I've also tried the Narnia books by C.S. Lewis just because I loved them as a kid and the language is pretty simple. The language is also pretty accessible in the Harry Potter books - and as a plus you can borrow them for free on kindle if you're an amazon prime member - UK, don't know if this applies elsewhere?
Getting away from fantasy, and also back into books originally written in German, I've been recommended Gut gegen Nordwind by Daniel Glattauer
-it's a romance all told in e-mail format - woman sends e-mail to a stranger by mistake and things go from there. Can't vouch for this one as I've only just started it but language wise seems OK and I'm guessing has a more normal vocabulary not including witches, elves, magic etc!
I'll let you know if it's any good once I finish!