Heavily researched and footnoted (the last hundred pages are references), it's an overview of the practical side of what's known about the microbiome and the best book I know of so far that summarizes most of what is known about the affect of microbes on various health conditions and brain states.

It's written by a clinician, not a scientist or journalist, so it's hard to tell when he's writing from opinion (e.g. his rants against GMOs) or whether there's a real scientific consensus on something (e.g. the role of bifidobacterium). ( )