Deciding on which genre or category your book fits into can be a difficult decision for any author. Of course the easiest way is to decide this before beginning writing but it is not always that easy as the book evolves during its creation. Yet this decision is important for anyone who is serious about its marketing as it is the means by which any bookseller, librarian or online marketer places it on their shelves.
The largest online seller is Amazon and, if you use this outlet, it will place a book into one of its categories if you decide not to do this yourself when completing its online submission form and sometimes it can get this wrong. The best way to seek this out yourself is to search for a book which is similar in subject matter, style and tone to your own and look at the category Amazon fits it into or how other booksellers and libraries place it in their catalogues. Concentrate on those which top the best seller lists and try to identify the key words used in searches.
This method is not necessary foolproof as I have found that sometimes Amazon alters the categories even after you have listed them using its KDP direct website. It may have even found a better slot than you first envisaged so make sure you keep this under review. Amazon allows authors/publishers a chance to choose two categories for its KDP publications so it is better to slot your book into two different subgenres to help prospective readers find it (see example below).