Final answer:
The correct unit rate to express the time to read a book is 1/3 book/hour, indicating that in an hour, one can read one-third of a book.
Step-by-step explanation:
To rewrite book as a unit rate referring to how many books can be read per hour, we would look at the options provided and consider that a unit rate describes the amount of one item in relation to one unit of another item. To convert the time it takes to read one book into a unit rate of books per hour, we would take the inverse of the time since we are given that it takes 3 hours to read a book. Therefore, the correct unit rate would be the number of books per one hour, which is Option 2: 1/3 book/hour. This indicates that in one hour, one could read one-third of a book.