One thing that the Volek-Phinney book really opened my eyes to was that there is a "dead zone" when it comes to how many carbs you eat. According to them, you should eat eithermore than 150 grams per day or less than 50 grams per day. If you're in the 50-150 gram zone, you are not getting enough glucose to fuel your brain, while at the same time not producing enough ketones to do it either. At best, you end up fueling it by your liver doing a lot of gluconeogenesis (converting protein to glucose), which may mean breaking down muscle mass.
Interestingly, V&P's "dead zone" is Mark Sisson's "optimal" zone... I would love to hear them debate this out.