The Cennini forum... excellent resource! The early stages of study and development of my skills came from this site. It costs only around $8 to join and well worth it!The Cennini Bible can be downloaded and I suggest u keep it and study it!
A stroke of Genius.... Everything portraits is here. Get on to their forum, wow, a wealth of info and huge knowledge base, the best on the web re - portraiture.
Everything Technical...this is a MUST READ for every oil painter! Study it from beginning to end!