Books and maps and online resources
I've rarely met a book that I didn't like, so I have a tendency to buy about forty-seven books for any trip -- each one has some tidbit that doesn't show up anywhere else. I tend to go a little berserk on the books (especially now that I buy a ton of them on ebay). They show up in drbs and drabs from all over the place, and I spend my evenings paging through the travel guides, historical references, and other reference books that show up .
Not only did I pick up the castle books from Mike Salter at Folly Publications (the best reference I've ever found, to be honest), I pertty much bought everything else I could find.
I have most of the travel guides, but my favorites are the Rough Guide and Footprint Guide. Chock full of information and packed with maps, suggestions for hotels, and detailed information about the sites and architecture. I have Frommers, AAA, Routard, and various other books, but I did primarily stick with the Rough Guide as our basic reference - it was amazingly accurate for times and prices.
Some links and useful stuff:
Booksellers
Amazon • Barnes and Noble • Hamilton Booksellers • Alibris Books • Abebooks • eBay
Travel Guide Sites
Frommers • Fodor • Rough Guide • Let's Go On-line • Michelin Guides
The Insiders' Guide • Falcon Guides • CNN City Guides
On-line Guides
Epicurious Travel • CD-ROM Travel Planners • Traveling Channel • Magun's Compass
Web Travel Secrets • Microsoft Expedia • Europe Through the Back Door
Books and Magazines
Armchair Travel Company • Travelers' Tales Books • Conde Nast Traveler
Web Surfer Travel Journal • Adventurous Traveler • Easy Going Travel Shop
Maps
Ordnance Survey Maps • International maps • Rand McNally
World of Maps • Intl. Map Trade Assoc. • MapQuest • TrailWalk
Travel Clothing
Tilley Endurables's • TravelSmith • Walkabout Travel gear • Changes In Latitude
Columbia Sportswear Company(r)• CampMor • The Territory Ahead
Coldwater Creek • The Outdoor Woman • Nebraska Clothing Company
Expeditions • Pacific Trekking • Eagle Creek • REI • G.I. Joes
Travel Stuff
Magellan's Travel Supplies • Travelers Emporium • TravelSmith
Walkabout Travel Gear • English walking sticks