At the least, two travel books are needed:  A good overall travel guide and a bird field guide.  Other information can be useful—where-to-bird site directions or how to tackle the Louvre—but often these can be studied ahead and/or appropriate section torn out, or photocopied and trimmed, thus avoiding lugging a library. 

However, increasingly, these options—especially the overall travel guide—can be in the form of an app, Kindle book, eBook, or other more portable solutions than the traditional heavy book.  So far, I’ve found the electronic versions to be ultimately portable, though a little more difficult to navigate within.  Holding a book or sheaf of stapled pages (remember, you can tear apart a book, it’s okay) and leafing through to the restaurant recommendation for the town you’re in or to the opening hours for the nearby museum is the easiest.  But for a little inconvenience, you can carry tons of guidebooks in your handheld device.

Bird books for most foreign countries, with plates and all their informational wealth, are still for the most part in the form of a traditional book.  But stand by, I’m sure that’ll change very soon.

Below, find an overview of some the major publishers, types, and categories.  Visit their websites for the options in accessing their information.  

>BIRD FIELD GUIDES AND WHERE-TO-BIRD:
Many parts of the world are already well-covered with complete species guides, and new ones are being published all the time.

Good source:  ABA Sales/Buteo Books

>GENERAL ADVENTURE:
This is the one travel guide that you need, and the Lonely Planet series is the king.  However, Moon Handbooks, Footprints, and Rough Guides are all acceptable.  Check the publication date before you buy - sometimes Lonely Planet guides lag behind a bit in updating, and then another might be a better choice.
>THE THREE F'S:  
Fodor's, Frommer's, Fielding's - all American publications, all geared toward the regular American tour/cruise traveler.  However, can be useful in initial itinerary planning for the non-birder - the country's highlights and must-do's are easily discernible.  
>EUROPE THROUGH THE BACK DOOR:
A must if Europe is your destination.  No one navigates the array of European travel choices with such savvy and humor as Rick Steves:
ETBD Website.   
>IMMERSION:
If you have room for an extra guide, consider one of these:  DK Eyewitness or Michelin Green.  Neither will help you get into the city from the airport or choose a hotel, but if you want visuals and plans of Mayan ruins (Eyewitness) or a detailed history of that Tuscan cathedral over there on the hill (Michelin), these will feed your curiosity and add a dimension to your destination.     
Birds and Books, Peruvian Amazon