It isn't released, yet, but I'm pre-ordering this book
Picture Perfect Lighting: An Innovative Lighting System for Photographing People: Roberto Valenzuela: 9781937538750: Amazon.com: Books based upon the author's previous work.
Kelby Training and Creative Live are both online resources that have resources addressing this subject. There are many more online resources such as
Cambridge in Colour - Photography Tutorials & Learning Community and there are videos on youtube, as well. (Look for videos with Cliff Mautner, a favorite of mine.)
I believe the best way to learn about portrait lighting is to learn from and work with a master of the craft. After that, work on your own with a mannequin and light sources with modeling lights. With modeling lights, you constantly see how the strobe/flash will fall on the subject, and can adjust the light source as you see the need to.
If you want to work with other than portraiture, get up early and study the light and how it effects the scene just before and the hour after sunrise. Take a nap and study the same for the hour before sunset till the light gets so low you can't shoot any more. (Those times are called "The Golden Hour" for more than just the tint of the light.) When you're shooting outdoors, turns around look behind your camera frequently so that you don't miss a light opportunity.
Read about light in painting, as well. There are masters' techniques from the old days that work really well with photographic portraiture, as well.
WM