Our most recent art project was Jackson Pollock mimicry. One thing you ought to know about Holly and Greta is that they know how to consume art supplies at the fastest rate possible and therefore they had no problem dumping a ton of paint on poster paper. They really enjoyed this art project!
Unfortunately there are not a lot of children's books about Jackson Pollock, but we did read Action Jackson. We also borrowed from a the library a book with photographs of his work.
I recently visited the Chicago Art Institute and my girls were so jealous that I saw works by Mattisse, Pollock and Georgia O'Keeffe in person! That enthusiasm for art makes all of this worth it. This is one thing I love about homeschooling!