Try taking a look around a few bookstores, and you'll most likely find similar books in the "Horror" section.

As a clue to the percentage of the market taken up by this genre, take a look at how much space in bookstores is taken up by the horror section.  Obviously this will vary depending on the size of the shop...  You'd have to visit a number of bookshops and take a rough estimate.

Alternatively, you could read through best sellers lists and see now many horror novels are in these.