My favorite travel books for Europe are by far the Rick Steeves series of books. Steeves' writing style is relaxed and fun. Rick Steeves is also a seasoned backpacker and his focus is to highlight those special "off the beaten path" sights and activites that many people never know about.

His books are thin and lightweight, the perfect size to fit in your purse or pocket, and very reasonably priced.

If you are visiting several countries, I'd purchase a multi-country book. These books have all of the highlights and are organized into categories such as "Don't Miss" and "If you have time"

If you will be spending several days or more in one country or city then I would recommend purchasing the book for that country or city which will give more indepth info.

Caveats:

Steves is all about "budget travel". Some of the accommodations are bare bones and you won't find very fancy restaurants in his series.

His maps should not be relied upon. They are hand drawn maps, often not to scale and containing the landmarks that he uses to get around. I recommend that you stop at the Tourist Info booth and pick up their free city maps for all the cities that you plan to explore