Identify the error in the sentence: This is one of those books that are difficult to read.

The sentence "This is one of those books that are difficult to read" contains a subject/verb agreement error. In this context, the phrase "one of those books" implies that the subject is "one," which is singular. Therefore, the verb should also be singular to maintain proper agreement. The correct form would use "is" instead of "are," resulting in the sentence saying, "This is one of those books that is difficult to read." This adjustment aligns the verb with the singular subject, ensuring grammatical accuracy. Understanding subject/verb agreement is crucial for clear and correct sentence construction, especially when dealing with phrases that might suggest a plural meaning.