What topics usually interest a reader!

Well written books, good plots, and the knowledge that the author really knows what he's writing about.

There's no shortcut, I'm afraid: write what gives you pleasure, and write it to the very best of your ability. You can sit down and say, 'Right - gothic horror sells well - I'll write that' - but if you don't know anything about the genre and don't like it and have no interest in it, that's the sort of result you'll get: flat, poor, dull, boring. It won't sell.

Write the story you want to read.

There are many genres to choose from. It depends on what types of novels an individual likes. I'm sure that authors would be writing frantically if the knew in advance what the next popular trend would be.

I suggest you look at the lists of best selling books.