Can anyone provide any suggestions on how to write a book that will sell? How do you find the right topic and making sure there is a large enough audience?

Go on Amazon and see how many books are in the space you are looking at and look at the sales rank number of the books if they are very low then you may want to consider another topic.

Check out some of the SEO tools that track how many times people search for the topic you are considering writing about - that should give you some indication on what maybe a strong selling topic.

I would go into some of the user groups and see what people are talking about you can get some good ideas there.

I think it is also important to look at the results i.e. how many pages show up when you do the search that will also give you a sense of the competition.