Step 1: Understanding the Question:
We need to find the grammatical error in a sentence expressing a superlative opinion about a book.
Step 2: Detailed Explanation:
Let us analyze the grammatical construct used in the first half of the sentence: "This is one of...".
The fundamental rule for the construct "one of the" is that it must always be followed by a plural noun or a plural pronoun.
The logic behind this rule is simple: you are selecting "one" single item out of a larger collection or group of many items.
Therefore, the group you are selecting from must inherently be plural.
In part (2), the sentence says "the most interesting book".
Here, the word "book" is a singular noun, which violates the rule.
It must be changed to its plural form "books" to make logical and grammatical sense.
The Correct Sentence is: "This is one of the most interesting books I have ever read."
Since the singular noun is incorrectly used, the error lies in part (2).