Take note, this book is not about basic english grammar. It shows how language has evolved and transformed with further usage. This book seem to tell us about the fine print we missed when we studied basic grammar because learning it has become boring and tedious.
Though the title suggest adjective as an exclusive topic, the book dealt on other parts of english grammar. In a fun way, the author showed how speaking a language changed an unacceptable basic english grammar to a regular use. An adjective modifies a noun or a pronoun, and an adverb modifies a verb, they could also modify against one another, i.e., they could be adjectival or adverbial.
A regular adjective is easily identified when it uses -ier, iest, to indicate grade. The use of -ly (slowly, moderately, etc.) on some words has become a common practice that were unnecessary because they never changed the meaning of a statement.
Quoting W. Zinsser, “Most adjectives are …. unnecessary. Like adverbs, they are sprinkled into sentences by writers who don’t stop to think that the concept is already in the noun.”