Just found this great website filled with Indonesian textbooks from introductory language study to social sciences to the arts to natural sciences!
They start at a very elementary level (they have first-grade books) and go up from there. I haven't explored thoroughly yet, but I've seen seventh-grade books, so I'm certain they go at least that high. There are literally hundreds of them for download. All you have to do is sign up, which is very hassle-free; name, e-mail, and city is about all they want. You'll have to fib and choose a location within Indonesia.
Anyway, enjoy, and don't forget to thank the Indonesian government for all this free reading material, if only in your head!