It's more about books, but it's a part of bookstores which is not part of the digital reading and shopping dullness:
I have been talking to a friend who is opening a bookstore. He had a much smaller one, mostly used and remainders, and closed it a few years ago to focus exclusively on internet bookselling. Now he is opening a larger, more general bookstore in a busy neighborhood.
He has been thinking a lot about categories. Since I categorize my lists, he thought I might be good to talk to, but he has quickly gone way beyond my 51 categories. In a physical bookstore you can have so many more categories and subcategories.
Having a physical bookstore is so different from being an internet marketplace seller. The books sell because of where your store is located, where you put books in relation to other books, where you put them in the store, how they look, what your staff says about them, what your customers are reading lately, or hearing from other customers, fixtures, signage, light.... This is why categories are so important to me, as a broker with limited sales tools. Before I send lists to most of my customers, I spend a lot of time categorizing them, indicating what's new since the list was last sent to the group of customers I am sending to, and what has sold recently. I also fill in missing data in all fields, but categories are the most important and time consuming. If the book was on previous lists I use the category and other data assigned to it previously, otherwise I look them up.
The difference between what my bookseller customers do makes it impossible for me to ever have a rock-solid set of categories that can be applied to every list. If there are 5000 titles on a list and 2 of them are biography and 1000 are history, the customers that look for biography will never buy the 2 titles, so I would probably put them under history, where the author or the subject might sell them. If I have enough readily identifiable science fiction on a list (always a challenge), I will put them in their own category, otherwise they go under fiction. If a customer just wants material on ancient history I have to search for everything from all categories that might apply. I don't have enough demand or supply to create such a category by itself, but for this customer I do the work every time. I have a bunch like this, so subcategories are an ongoing project. There are so many situations like this that I can't think of them all; they occur with almost every list.
I have 51 categories I apply to my lists. I used to have more, but I have consolidated some. But in a bookstore you can have many categories tailored to what your customers are looking for. You can have a local history or interest category.
Mistakes sell books in bookstores in a way you can't sell books on lists, physical or digital. You will sell more fiction if you occasionally put the novels next to the same author's work in non-fiction sections. Put something erotic in the parenting or cooking section. A dated home repair book in the antiques and collectibles section, or with the grilling books. A Bobbi Brown beauty book with the yoga books. What a mistake, the customer thinks as she picks up the book to page through it.
Create flows from window to tables to shelves to other categories. If a bestseller is tied in to a movie, similar subjects may sell more if located close by.
Years ago a used and rare dealer I knew bought a large lot from another dealer who was closing down after many years. He thought he was getting mostly literature by authors of the Hudson River Valley, and books about the area, but, due to a mix-up, it turned out to be a few thousand books on automobile repair. There were manuals, annuals, catalogs, paint swatch books… All were dated, some by decades. This was not what he had bargained for. He had overpaid for books which he was sure were unsellable.
He left them in cartons in his storeroom for several months, but when an ice storm caused a leak in the roof, he put everything onto shelves in the store and sorted it out as best he could. He now had a large auto repair category. For a few more weeks the books pretty much sat there, being ignored, a few selling online. Then one of his customers turned out to be the owner of a car repair shop in a nearby town. This guy had been shopping at my friend's store for years, buying crime fiction off the sale table. He bought about 10 items from the automobile section that day. Over the next few weeks his mechanics started coming in, buying a few books at a time. They then brought their kids to shop the children's section. Eventually a collector showed up and bought most of the really old stuff.
There is no way to know what will happen when you buy used books, and no way to know how well you know your customers until you put something in front of them you thought would never sell.
I got turned around again. What began about categories became more about merchandising. But categories are a big part of it all, including how I love and understand what I do in this business.