I say binoculars, but Tom McIntyre's The Field & Stream Hunting Optics Handbook actually covers shooting glasses, rifle and telescopes, binoculars, and rangefinders. And flashlights.
Before turning to binoculars, however, McIntyre devotes chapters to the eye itself and to shooting and prescription eyeglasses for the hunter ("You need protective [lenses]—think hunting with Dick Cheney.")
The Hunting Optics Handbook is moderately technical. You need to know some formulas to compare the "twilight factor" of two different spotting scopes you are comparing, for example — how well they work under low-light conditions.
But along with the useful, contemporary information, McIntyre packs in a lot of history (of lenses, iron rifle sights, and marksmanship), and that history makes this book something more than a technical treatise.
He suggests, for instance, that General Custer's bad day on the Little Bighorn in June 1876 might have been partially due to his "low-powered, undoubtedly high-dispersion-lens field glasses."
After trying to study the terrain through the mid-morning heat shimmer, Custer allegedly turned to scout Mitch Bouyer and said, "My eyes are as good as yours, and I don't see any Indians."
Supposedly the same glasses turned up on one of the Lakota corpses at Wounded Knee a generation later.
Since serious purchases of spotting scopes, etc., can run well into three figures--or more--doesn't it make sense to spend $15 and a few hours educating yourself first? And then keep The Field & Stream Hunting Optics Handbook on the bookshelf the pleasure of reading about a medieval pope's eyeglasses or Hiram Berdan's sharpshooters.