What kind of really good books often wind up in the clearance section?
Books from another generation.
True with science fiction and true with cookbooks.
Cooking, much like clothing, tends to run in fads.
Try being the author of a high carb and artificial color cookbook anywhere.
But that was our life 40-50 years ago. (Jello molds?)
OK maybe not the great cookbooks but forgotten ones.
I once found our copy of the Joy of Cooking for $3, many many years ago.
And we seldom open it anymore.