I really like Gamblin's Asphaltum as it is so "Old Masterish." I plan to keep it in my palette forever.
It is a very clear, warm, and delicious brown color to glaze with. Until I began using Asphaltum, I was using raw umber to accomplish my warm brown glazes.
Of course, I'll keep using the raw umber for many things, but nothing really is like the even warmer tone of Asphaltum. I find that it really gives a lot of life to deep shadows. I think that you're going to love it for sure!
And I agree, Joseph Sheppard's book is the only written instruction that I have ever found that helped me make the giant step in learning to paint more like I really wanted to.