When you upload a self-published book to KDP, Amazon gives you a choice of categories. Many authors simply choose those they feel are most applicable to their book. This isn’t necessarily a bad approach – Amazon tends to reward transparency – but it’s often possible to make categories work harder.
How? By choosing a category with less competition, you can increase your chances of ranking higher in it’s top 100 books. Make it into the 100 or higher and your book will theoretically gain more exposure, since readers may browse by category and see your book in those all-important top 100.
Generally it’s best to avoid the mega large categories like ‘general fiction’, or ‘romance’ unless you can be sure of selling over 1000 books in a single day. Anything less and you’ve no chance of making it into the top 100 charts in a large category.
It’s a law of diminishing returns though. Choose a tiny obscure category and not many people will look for it. Pick a category that doesn’t reflect your book and you risk annoying readers and getting bad reviews. The best strategy is to pick one larger category – say 30,000 books and one smaller 5-10,000.
Sneaky tip – Get your book in more categories than the standard two
Kindle and paperback book categories are slightly different. If you see a category you like in the paperback (BISAC) categories that isn’t available on Kindle, email KDP and ask for your book to be put in it. They will gladly oblige and this gets you in one more category than the standard two.