My road trip and RV travel collection is about three feet of shelf space, at least the part of it I keep stacked on and in the cabinet next to my desk. Not all of it my choice, my sisters started buying me road trip books as gifts when I retired. Some of them I've found browsing through used book stores, a market that seems to have since moved to Internet marketing.
It helps to open up beyond RVing to road travel generally. My RV-specific books are kind of old, and what is RV about them gets dated quickly. Some may have newer editions, others not. For what it is worth, in that category:
"RV Vacations for Dummies" by Harry Basch and Shirley Slater
Frommer's "Exploring America by RV" by Shirley Slater and Harry Basch
"Great RV Trips" by Charles L. Cadieux
"RVing Basics" by Bill and Jan Moeller
"The RVer's Bible" by Kim Baker and Sunny Baker
Then the general road trip books, the first being the best I've found for planning my travel adventures, while the last three are coffee table books that might build an interest in travel:
"Road Trip USA" by Jamie Jensen
NGS "Scenic Highways & Byways"
Reader's Digest "See the USA the Easy Way" for loop tours
Reader's Digest "The Most Scenic Drives in America"
NGS "Drives of a Lifetime" (a global bucket list of 500)
Then there are specific route (or region) guides, the first being the best I've found for the historic US-66 (I have several others):
"EZ66 Guide for Travelers" by Jerry McClanahan
"Native Roads" by Fran Kosik
"Scenic Driving Texas" by Laurence Parent (a Falcon guide, others for elsewhere)
"Adventure Guide to the Alaska Highway" by Lynn & Ed Readicker-Henderson
"Scenic Driving Alaska and the Yukon" by Erik Molvar (another Falcon guide)
And road trip guides that tell a story and suggest what to look for but don't necessarily show you the way, like:
"Route 66" by Michael Wallis
"The Lincoln Highway" by Michael Wallis
"The Complete Route 66 Lost & Found" by Russell A. Olsen
"Adventure Guide to Oklahoma" by Lynne M. Sullivan
"Roadside History of Oklahoma" by Francis L. and Roberta B. Fugate
First three are coffee table books, last two are more useful when traveling, providing information about the places you are going through.
Then more general regional and local guides not focused on road tripping, rather about the destinations more generally:
NGS "Complete National Parks of the United States" by Mel White
Sierra Club Guides to the National Parks (separate guides regionally)
"The Essential Guide to Black Canyon of the Gunnison National Park" by John Jenkins (there are others in this series by the Colorado Mountain Club).
"The Colorado Plateau" by John A. Murray
Discovery Channel Insight Guide "American Southwest" (others in the series)
Some special interest travel guides:
"Watch It Made in the U.S.A." by Karen Axelrod and Bruce Brumberg
"Road Food" by Jane and Michael Stern
"Eccentric America" by Jan Friedman (a Bradt Guide, more in the series)
"Weird Texas" by Wesley Treat, Heather Shade and Rob Riggs (others in series)
"Tony Hillerman's Navajoland" by Laurance D. Linford
"Roadside Geology of ..." a series in which individual books might be regional, by state, or in some cases by highway.
Books about skills or activities one might associate with travel:
NGS "The Ultimate Field Guide to Photography"
NGS "Field Guide to the Birds of North America"
NGS is the National Geographic Society, one of our biggest publishers of books for and about travel, history, geography and cultures. This is the time of holiday book sales at the NGS, according to the junk mail they constantly send my because they know I've bought a lot of their books.
Discovery Channel, Frommer's, Falcon Guides are also good places to start. Used book stores, "charity" resale stores are another good place to look. One of my treasures is a 1982 edition of "Book of British Towns" published by the Automobile Association, which cost me just one pound. Library and fund-raising used book sales also yield some travel treasures, but the used bookstore guys tend to get to the bigger sales early, so inventory gets somewhat picked over.
My regional focus is the Southwest, because it is what interests me (geology not buried under vegetation). You can put together collections for other parts of the U.S. just as readily, looking at the catalogs of the publishers of these travel guides.
For the country as a whole, I think Road Trip USA is a good start for planning because it has six north-south and four east-west trips laid out in detail, most of them coast to coast or border to border. It is not necessarily a book to generate interest, however. The coffee-table books work better for that.