There are many great travel guidebooks out there, for just about any place you might want to visit. However, they tend to cover a whole country at a whack, which means (unless it is a very small country) there is lots of material in the book you dont need once you have planned the trip and are on it.
For instance, on my trip to Peru last year, I only visited Lima and Cuzco, so the rest of the book on Peru I had was just dead weight while on the trip. I could have torn it apart, discarding the unneeded chapters. But thats messy and I hate defacing a book. And who knows, I might go back there someday.
How about a line of travel books produced that have a CD-ROM bound into the back, that contains the contents of the book in machine readable form. Provided with it will be a simple program that allows you to enter your itinerary, and it will select all the portions of the book that continue to be relevant to your trip.
Also handy would be a function that would suggest a convenient order in which to do things. For instance, suppose you want to visit three different museums that are spread out, whats the best order to do them in given their operating hours and locations relative to your hotel?
These tidbits from the book can either be printed out as a daily itinerary, or downloaded to your handheld computer for easy access while on the trip.
(I am aware that both Fodor and Frommers have recently introduced PDA versions of some of their guidebooks. These applications seem quite focused on choosing restaurants and hotels, and ignore the rest of the material in a travel book. They also dont have a print function for those with a computer but no PDA).