To find out how many books Micaela and her three friends collected altogether, you need to multiply the number of shelters by the number of books each shelter received.
There are 4 shelters, and each received 18 books:
\[ 4 \text{ shelters} \times 18 \text{ books/shelter} = 72 \text{ books} \]
So, Micaela and her friends collected a total of 72 books.
The correct answer is 72.