According to yesterday's post, people are looking for one of two kinds of books: books that help and teach, or books that entertain.
Eat This, Not That (now faded from popularity, like most fads) was an overnight sensation, because everyone wanted a book that would tell them how to lose weight easily.
The Help gained popularity so fast, it was a movie before some people had finished reading the book; a moving, compelling account of the hardships of being African American house help in the south in the 60's.
One helps, the other entertains. If you can figure out what people want to learn, or how they want to divert their minds, it's very likely that you can write something that thousands of people will buy.
Study your audience, then appeal to them. After all, they're the ones who are buying your book.