Just off the top of my head, and certainly without referring to any textbook, I'm thinking that if one is referring to a noun in the singular, one would usually use "a" or "the" (depending on whether it's a generic reference or a specific reference). In the plural, referring to the noun in general, one does not include "the" (which again implies specific books).
Reading a good book will help you relax.
Reading the book I gave you will be fun.
Reading books will help you relax.
Reading the books I gave you will be fun.
Usually the plural of "a/an" is "some," but that is restrictive to only part of the group.
A dog is bigger than a cat.
The dog is bigger than the cat.
Some dogs are bigger than some cats.
Dogs are bigger than cats.
The dogs are bigger than the cats.
Each sentence conveys a different message. #1 and #4 are generic. #2 and #5 have specific animals in mind. #3 is also generic, but it is more limiting.