Check your local library. Check your local REI store or outdoor store or bookstore. Check the bookstores at national parks/ monuments if you're close to one. Lots of info out there for Colorado.
When we lived in Fort Collins, the Forest Service office there had a lot of hiking info- that's been several years, though; it may be online now.

Also look at the "climbing" books. I've got a little book by Gerry Roach on Classic Hikes and Climbs in Rocky Mountain National Park. I'm not a climber, but that book includes a lot of walk-up routes as well, and is handier than it first appears.