Sand the edges of all of the three pieces of wood that you'll make the book holder from. This will make the wood easier to work with. In addition the stain or paint will adhere better to smooth surfaces. Wipe all the sanded pieces using a dampened towel to get rid of the excess sand or wood bits.
Nail two 12-by-20-by-1 inch pieces at a right angle along the 20-inch length. Ensure that the 1-inch width of one pieces is nailed to the top of the other piece. The produced "L" shape will become the support for the book holder with the two open ends sitting on the countertop, desk or shelf.
Nail the 3-by-20-by-1 inch piece of wood about an inch from the bottom outward facing edge, with the 1-inch edge against the board, lining up the 20-inch length of both pieces of wood.
Stain or paint you wooden book holder as you see fit.
Let the stain or paint dry completely before you can use it. A book will rest on the small piece of wood, the holder leaning back on the two legs of the "L" shaped angle.