Jason Isbell is one of my absolute favourites and - whisper this quietly - but he is genuinly up there with may of the greats in my humble opinion. Stylistically I guess he cuts across rock, country, alt country, americana, Southern, folk but you're absoultely right its better to just call just damn good music.
I appreciate I'm a little late replying but came upon your post running a search on 'Jason Isbell'; anyway just in case if youv'e not your found some of his other work take a look at his 'South Eastern', 'Here we rest' and 'Something more than free' albums which are some of his more recent albums to much critical acclaim. Equally amazing is a solid and vast back catalogue of work with his old band The Driveby Truckers (amazing stuff, I particularly like The Dirty South and Decoration Day)