Nancy Clark's book is excellent. I actually have three versions. Even her recipes are good!

Just glossing over "Fueling during and after exercise" says:

100-250 calories of carbs per hour after the first hour
Mix it up--use food and fluids to get a variety of fuel
Experiment to see what you can tolerate
"Keep in mind that too much sugar or food taken at once can slow down the rate at which fluids leave the stomach and become available to replace sweat losses. Be more conservative with your sugar fixes during intense exercise in hot weather, when rapid fluid replacement is perhaps more important than carb replacement........."
Weigh yourself before and after to determine if you are consuming enough fluids.

During our first century, I actually set a timer for every 45 min, so we didn't get behind on intake. I can't say we ate the right things, but we ATE! If I rolled into a stop and one bottle wasn't empty, I knew I wasn't drinking enough (stomach sloshing is bad--I used to get that running!)

Marathon books are also great sources of info. For some reason there just aren't as many books about endurance cycling! Although runners stomaches tend to me more sensitive, the info is still applicable. Try Marathon by Galloway.