The Lonely Planet guides are always, I find, so reliable. (This being said, I've only bought them USA destinations.) Now that I'm planning a four week road trip next year, I had to buy this.
Each state has it's own guide, with the typical list of recommended sites, places to eat and of course accommodation for varied budgets.
There's a hand full of road trip recommendations... all very tempting. This is great if you really don't know where to start. There's also the usual guide about how to to get about, car rental, flights, taxis, etc - essential for your planning.
I've booked up about half my trip already, and most of that has been done after reading sections of this book.
It's incredibly helpful, and I whole-heartedly recommend it. (If for some reason it all goes wrong, though, I'll be back on here telling you otherwise!)