If you want to write a book I think it would be okay to assume you also want to sell it. This is where it becomes important to choose the right kind of book. So lets look at a few things to consider to get you on the right track.
If you are going to approach a publisher research them first and find out what kinds of books they publish. Gather as much information as you can about them and keep it organised. Familiarise yourself with what is selling, most bookshops list the top ten sellers. Check the publishers of these books and make sure you include them in your research. Stay on top of what the publishers guidelines are as these can change. There is no point going to the trouble of writing something if publishers aren’t interested. Even if you choose to publish yourself, and this isn’t difficult, you still want to know it will sell.
When deciding on a topic you know is popular, for example wizards, make sure you have a new, fresh and original approach.You also need to decide on fiction or non-fiction, what age you will write for and whether or not you will do one off stories or a series based on the adventures of certain characters. Most of these decisions are yours to make but even so to be successful you must have a plan and a basic outline of how the book/s will unfold.
Be very informed on who the readers of your book will be, what they enjoy and are interested in, the language they use amongst their peers, their views on life and what their concerns are. All these issues will guide you in your choice of characters, the length of your book, and your writing style.
Choosing the right book to write will also depend on you and how you fit with each possible avenue. You must write to suit the reader so find out where your strengths lie, learn what the market is telling you and get to know publishers preferences and guidelines.