In grammar, a predicate nominative is the first noun (or a noun phrase, in this case) that comes after a linking verb. If you take a look at the example above, you will see that the linking verb is <em>IS </em>and the first noun phrase that follows it is <em>one of Mark Twain's most famous books, </em>which means that the phrase is a predicate nominative.
The subject of the sentence would be <em>The adventures of Tom Sawyer </em>rather than the second phrase.
