We have a store called Ollies around us. Like Big lots with overstock and blow-out items. The book section will have cheap woodworking books, which I have several bought for $1.99, 2.99, 4.99 etc.
Also, go onto Amazon.com. select books and search for router / woodworking books. Look at the used books, and you can buy used for way less than new. I buy a lot of used books, and I cannot say I ever got stung on the quality based on the amount paid. They tell you the condition of the book, if it has margin notes in it, etc.
Bill Hylton's Power-Tool Joinery (Popular Woodworking) by Bill Hylton (Jul 1, 2005) is used - $1.44 + 3.99 shipping.
Country Pine: Furniture You Can Make With the Table Saw and Router (Hardcover) $1.78 + 3.99 shipping.
Country Pine Furniture Projects: 32 Classic Pieces to Build for Your Home (American Woodworker) by Bill Hylton, Sally Onopa and Mitch Mandel (May 1, 2008) $0.25 + 3.99 shipping in "used acceptable condition" or $7.89 + free amazon shipping in "used-good" condition.
Success with Routers: Techniques & Tips by Fred Matlack Bill Hylton (2000) used - $4.00 Amazon prime.
There are lots of good inexpensive books out there. just keep looking on the internet.