The amount of information in any guidebook is way more than my attention span.  Further, I forget many of the helpful specifics (like which of the Pompeii train stations is closet to the ruins) by the time I get there.  You don’t want to be lugging multiple guidebooks with you, so if you buy a bunch at least some of them are staying home.  I buy the country specific books in situations where the additional culture and phrase book information or information about out-of-the-way locations is important, such as:

[blockquote]I’m working there or will be staying in an apartment or other non-tourist accomodations.
I’m going to be there for 2 weeks or more and will be off the beaten tourist track.
I’m going to be staying with, or otherwise spending significant time, with locals.
They’re in the clearance bin at the bookstore and cost less than $5.
[/blockquote]
If you find you want more specific information you can always buy English language guide books specific to a city, a particular attraction or a country after you get to Europe.  They’re available at “tourist information,” at large airport or train station bookstores, at a book store or news stand that carries English books and papers (in virtually any major city in Europe and lots of smaller ones) and at any high profile tourist venue.  

Unless I’m in one of the situation described by oldlady above, I go to a local library (well, not really, to a local Borders store) take several guidebooks and make notes to take along.
Useful info for any destination normally comes from two-three sources and fits on just a few pages. This way it does not add much bulk, and lets me see right away what has caught my attention at the first place