For this category of the Sapphic Book Bingo, you shouldn’t have any problem finding enough books to choose from. Romance novels are the most popular genre of sapphic fiction, so there are plenty of books with the word “heart” in the title. Of course, you can also read a sapphic book in another genre, as long as the title includes the word “heart.”
The word heart can be part of an open compound, e.g., Heart Trouble, or a closed compound, e.g., heartfelt. It can be single (heart) or plural (hearts).