I rarely take photos of work in progress, but in the case of my painting for Paul Guinan and Anina Bennett's Boilerplate book, I needed to be able to show where things were headed. I took snaps of the pencils, midtones, and the final piece. This was done in acrylic, like most of my recent paintings.
If you haven't picked it up yet, Boilerplate: History's Mechanical Marvel is an incredible piece of work, with some of the best photo manipulation you'll ever see. Check it out.