I'm looking for something like Duolingo, but for Latin. (By the way, it is a bit upsetting that hardly any of these sites include Latin as a learnable language; although I suspect that it is because the teaching methods of these sites rely heavily on the spoken word.)
I'd be very happy if any of these sites are up to snuff and not obscure, vocabulary sort of resources.
As for the books, they needn't be free, but good, established books for learning Latin.
I am almost exclusively interested in the written form, and not the spoken version. This is to aid in my reading of philosophy.