The book was mysterious, scary and full of humor?? Is that a parallel structure?

It is not a parallel structure.

Explanation:

Parallel structure or parallelism is the use of words in such a way that they are presented or used in the same pattern. In other words, words should be used in their same forms, such as using "-ing" or adjectives or the same form of the verb.

In the given sentence, the parallel structure will be between "mysterious, scary and full of humor". Looking at the individual words "mysterious", "scary" and "full of humor", the first two are adjectives while the third is not. And so, to make them parallel, we have to change the third into the same form, i.e. into an adjective. This means that the given sentence is not a parallel structure.

Thus, the final sentence will be

The book was mysterious, scary, and humorous.

or

The book was mysterious, humorous, and scary.

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