You see the bar below this edit page? It has a little button saying "Add category". Click it. Now type a name of a category that you think the book can go in. So if the book is Historical Fiction, put "Historical Fiction" (without the quotes"), then press enter. Done. One category added. There are more so continue to type as many as you want with the same technique! A full list of categories can be found at Book Categories (after saving this page!).