I bought the Apple Keyboard cover. Covers both front and back (and includes a keyboard obviously). I've never been a fan of cases or covers (my iPhone has been naked since I purchased it) so I'm probably not the most qualified person to ask about cases :).

My 12.9 is perfect for reading and annotating music sheets. Bought an used one mainly for that. The large screen size is close to a regular paper.

I have a ztotop one for the new 12.9. Works well, a bit heavy, maybe 50% more than the official one. But an 8th of the price, and otherwise works beautifully.