If you can find one, the darkroom printing book by the late Gene Nocon is very,very informative. I managed to get one 2nd hand after it had been out of print for a few years. This is the book that introduced me to 'zone' printing and the technique of using a blue filter over the enlarger grain magnifier to get the picture truly in focus. (Based on the theory that B&W paper is blue sensitive and not affected by other light)