I have an Aspie son, so know how it can be a tough struggle between feeding their reading habit and finding age (and maturity) appropriate books.
I was going to suggest a Nook, if it's in the budget, that way you're able to download a "teaser" and see if you like the books, and they are always available.
Some of the series my son read/s is the Ranger Apprentice, Eragon, Barthemous Trilogy (sorry if that's misspelled!), Atemis Fowl. There are a ton of great ideas already. I'd also recommend the Little Women series. I was about that age when I read those books.
At 12, my son really developed an interest in non-fiction as well. Biographies and Historical type books. How it's made, modern marvels kind of books. Guiness Book of World Records, too.