Very mainstream so you may have already read them but Born to Run and Eat and Run I found both to be great reads and with some great knowledge. Particularly found the ongoing diet debates through both interesting and Eat and Run has the added bonuses of some recipes thrown in for good measure.
In reply to JMGLondon: Why We Run by Bernd Heinrich. Entertaining mix of fuelling experimentation and world-class ultrarunning where the former might be just wacky (and possibly still is!) if he wasn't also a renowned biologist...