Painting on Location: Secrets to Plein Air Painting, is an excellent presentation of his working methods, with many large reproductions of paintings that don't appear in previous books on Curtis's work.
His subject matter includes primarily coastal views, city scenes, and rural landscapes—no portraits or still lifes.
Curtis typically works fairly small in oil. Most of his paintings are around 8x10 or up to 12x16 inches. In his book he discusses how speed and confidence are affected by the scale of the work, and by other challenges such as limited time and changing light.
The book gives equal weight to his watercolors, which makes it valuable to specialists in either oil or watercolor, or to artists who want to branch out to a new medium. There are a few step-by-step sequences, photos taken of the motif for comparison, and photos of the painting rig.
Here's a video shot on location that shows some of the watercolor techniques in action. Curtis confidently drops color into wet washes in the early "ghost wash" stage. As he says, "Observe once, observe twice, and dive in."
(Video link) And here's another video trailer showing his oil techniques.
...also available from the North Light bookstore