Jessica,
Rick's books tend to focus on the sites (historic and otherwise) that most North Americans are typically interested in seeing. As a result some lesser known but very nice areas are only covered minimally or not at all.
Since Rick's books are updated every year, there's a somewhat pragmatic reason for not expanding the books to cover more areas and more cities. In order to update the books, either Rick or one of his Guides has to visit the lodgings and other facilites in each area at least once a year to make sure the information in the books is current. This takes time and of course money (this sort of thing always comes down to money!). Even with a Staff of about 70 in Edmonds and numerous European colleagues and Guides, I'm sure this is an enormous effort!
Incidentally, Rick is currently in Europe doing Guide Book research, so there will be some new information in next year's editions.
If I'm looking for information on travel in a particular area, and the details aren't in Rick's books, I resort to the internet and other Guidebooks such as Lonely Planet, Let's Go, Rough Guides or Frommer's. This tends to provide a fairly good "composite" of information for planning trips.
Cheers!