A unique eBook about unjustly neglected string quartets is available for free from:
Yes I wrote it :) and I'm sure there's nothing else like it. Unlike a normal book where you'd just be reading about a piece, it contains listening links to YouTube videos so you can hear a work right away.
It's amazing how many famous composers wrote quartets which are inexplicably neglected (eg. Schubert's 8th string quartet, as beautiful as the Trout Quintet).
But of course there are unsung composers like Norbert Burgmüller, whom Schumann ranked alongside Schubert as a genius robbed from us by early death.