I really love these books too, and can't wait to pick up the ones that just came out.
But be warned - they are NOT d20 Modern sourcebooks. They are more like stand alone games similar to the mini-campaign settings in the old POLYHEDRON flipsides of DUNGEON Magazine, or FFG's Horizon line. All you need is a PHB to play. They work together, but you don't need all the books in the series to play, and some of the books have some overlap/duplication of material (crunch and fluff).
They also include a huge proliferation of new, highly specialized skills. Some of the uses of these new skills would make excellent new uses for existing d20 Modern skills, however.