Am currently overhauling my book blog! I stopped using it because I made it so difficult for myself to use it with the layout I had. So, fresh start, clean minimal look, and hopefully a better track record of reviews this year!
On anything you like! I just try to keep a book blog as I like writing down my thoughts about books, but people make blogs about all sorts of things! That can become a nuisance to maintain if you don't always feeling like updating them, though.