Simmons says that the Saudis are a lot closer to running out of oil than the world realizes, and that Saudi production is at unsustainably high levels right now. This certainly contradicts the ever-more-optimistic Saudi claims about reserves and production capacity. I've wondered what was behind the Saudi claims, though I'm no expert: My actual thought was that the Saudis were trying to discourage exploration elsewhere, but the other possibility is that they're in a desperate endgame.
Simmons' book is blurbed by bigshots, but I hope it's wrong. It's certainly another argument for doing what we know we need to do anyway, which is to increase efficiency and find other sources of oil, and energy generally.
UPDATE: A skeptical take on the oil-shortage scenario, here, and here's a CSIS report on Saudi oil reserves that specifically responds to Simmons' claims and finds them wanting. Beats me, but as I say, it's pretty clear what we should be doing.