distinct group of entries

i'm trying to retrieve a list of books, one book from each category, based on popularity...and i'm not sure how to go about doing it.

example:
Categories:
E - Educational (500 records)
M - Mystery (500 records)
R - Romance (500 records)
A - Adventure (500 records)

so in order to get the list of books with the most reviews:
select *
from books
order by reviews desc

..this will return 2000 records of course...now if i want only the most reviewed book from each category, how would i go about doing that?

thanks! 🙂