Seeing as he has written more the 43 other novels then the DT series I'm sure you could find something to read.
Even if you are not interestsed in anything but DT there are more then a few that have tie in's to DT. Check out Bev Vincent's The Road to the Dark Tower for them all.
The Talisman and Black House are 2 biggies, The Stand also, even The Eyes of the Dragon. Lots more to read from him.