The Fairburn System of Visual Reference is an impressive collection of photo reference. The nine books in the system are divided in three set, Children, Faces and Heads and Figures and Hands. Each book has thousands of images of people (activites) photographed at several angles.
I was surprised to see recently that these books are still being sold (Figures and Hands is out of print). More surprised to see them sell for $200 a set!
I bought my complete set many years ago. I've had mixed feelings about the investment over the years. The books were published in the 70s and the "models" look like rejects from Welcome Back Kotter. Bell bottoms and leisure suits can ruin a reference photo.
This collection of sketches is drawn from the Fairburn System. The books aren't perfect, but 25 years later, it's still the best thing out there.