(With all due respect, Kai.) You guys who think that you can cheat by adding a little lactic acid don't know much about the complexities of controlled (if you can even call it that) souring. I have been experimenting for about 5 years and I can tell you, it simply ain't that simple! A lot of (hopefully) pleasant, complex flavors are created during the aging/souring that simply can;t be recreated by adding a bit of acid.
BTW: OP, the book you need to get is, without a doubt, is "Wild Brews" by Jeff Sparrow.