Schlossberg is my go to guy. Who's your fav?
For now it's Max and Arbans. Check out Eric Bolvin's, "The Arban's Manual." It's cheap and will take you into JB's stuff with some sense behind our efforts.
Nothing helps as much as sensible practice and time.
60 days in, my tone "bucks up." The little things. Core, timbre, and the "ah ha's." After that, it's technique and facility.