In order for your books to do well even as a hobby, you should have a topic that you are passionate about. This will gradually increase the profit you can make because people want to read books written by experts or those who have "been there and done that." Determine which areas you can be profitable at, based on your experiences and the areas you think others would be interested in. For example, as a human resource manager for the last 20 years, you could write an excellent book on how to get hired and people would respect you for it.
Topics and Passions
In the area of fiction, there could be any number of topics or stories to tell. Often, in this genre especially, it is important to make the story realistic in some way. For example, if the story is one that real people can relate to, even though the story is about aliens, they will relate to it and will appreciate the book that much more so.
As a Hobby
In order to publish your books, you should consider self publishing especially if this is the first book you have written. Many do well in this area since you do not have to have the approval of the publisher to have the book published through them. Rather, you utilize their services to do the publishing for you, which may include the editing, layout, design work, and ultimately the printing of the book. When these factors come together, the end result is a book you are proud of, in print.
Self publishing companies are available for nearly all genres of fiction as well as nonfiction. You can find these companies online, with services easily made available to you right from their website. Learn about how the process works, the various packages of services you can purchase as well as the overall benefit of publishing with one company over the others. Many publishers are passionate about the books they publish and therefore will help you to publish your book, or books, in such a way as they will do well.
Publishing as a hobby is something you can do. With a bit of talent and something unique to talk about in your books, you could be writing books and making a profit off of them. Most hobbies cost money, but book publishing could help you make money.