Hint: Inter-Library Loan is amazing.
Here's a list of books I've found helpful, either for finding places to go, or in understanding the places I am in.
Guidebooks and Maps
Hiking Missouri by Kevin Lohraff
Ozark Trail Guidebook by Margo Carrol and Peggy Welch
Buffalo River Hiking Trails by Tim Ernst
Arkansas Hiking Trails by Tim Ernst
Ozark Highlands Trail Guidebook by Tim Ernst
Trails & Treks... by Kelly Frey
The Best In Tent Camping: Missouri by Steve Henry
Missouri Ozark Waterways by Oscar Hawksley
The MDC atlas (online)
North Missouri Mountain Biking, Hiking Guidebook by Tim Riekena
Hiking Iowa by Elizabeth Corcoran Hill
Hiking Guide To Kansas by Catherine Hauber and John Young
Hiking Trails of the Oachitas & Ozarks Maps for sale
Travelogues, Memoirs, History and Culture
Way Back In The Ozarks by Howard "Ozark Monk" Hefley
Stars Upstream by Leonard Hall
Rivers To Run... by Larry Dablemont
Listening Point by Sigurd Olson
"How can a man walk God's trail, if he does not see God's markings?" - Mincaye, Waorani Indian, Ecuador