There really are a lot of good choices at MK. I'll just mention some we have enjoyed.
Since you say "book" I assume you don't mean counter service, but counter service may be necessary to get a "reasonable price". If so I suggest Columbia Harbour House, located where Fantasyland and Liberty Square meet. It has surprisingly good food for counter service, and is an alternative to the endless burgers and fries you get at other places. The seating is broken up into smaller separate "rooms", and can be more relaxing than other counter service spots, especially if you sit upstairs.
Cinderella's Royal Table (the castle) is well known for its character breakfast, but is a great spot for lunch. It is no more expensive than most other sit-down places at WDW; when we went there in 2002, we paid well under $50 total for 2 adults and 2 children, less than we spent for any other sitdown lunch! And we got a window table!
Crystal Palace is a fine character buffet with Pooh & Friends. I recommend it, but it has the disadvantage that you'll pay $10 for each of the children, and it can be a little hectic like any character buffet.