What noun is the appositive describing in the sentence: "The book Jerome was carrying, a dictionary, fell into the mud."?

an appositive is a noun or noun phrase that renames another noun before it. often, it helps to explain or identify the noun.

the first noun here is the book. the word 'dictionary' renames the book, and makes the reader's knowledge of the type of book more specific.

the punctuation around an appositive depends on the noun before it. if the noun is too general on its own and does not distinguish a particular object, then the appositive does not need any commas around it.

e.g. 'the president John F Kennedy was highly popular.' without the apposition, the same sentence 'the president was highly popular' would be extremely general; the apposition 'John F Kennedy' is essential, and so is not surrounded by commas.

however, other appositives, such as the one in the question, are not necessarily essential. the sentence 'the book Jerome was carrying fell into the mud' would be a sufficient description for the reader to imagine. the apposition 'a dictionary' is not essential for the reader, and so it is surrounded by commas.

when an appositive is not essential, it may be easier to recognise with the commas around it.

The appositive "a dictionary" is describing the noun "book" in the sentence.