There are some questions in this life that we never can answer properly. Fortunately, the chicken or the egg question about the Rainbow Six game and book that's gone around the block in this space a couple of times in the past couple of days is not one of them, as Red Strom Entertainment's Jeff "Mad Samoan" Wand, modeler/artist on Rogue Spear, sends along what's hopefully an authoritative explanation of which is the cart and which is the horse here:

The book was based on the game. The original concept for the game was conceived in a brainstorming session with Clancy. The original idea was that it be a 3D first person SWAT game, but Clancy threw in the idea of taking it on a grander scale and giving it an international antiterrorist angle. Some time later after the brainstorming session, he decided it was such a great idea that he'd make a book out of it, tie it in with the game, and maybe someday, a movie. Hopefully starring me.