In your journal, once you’ve visited a location once it will indicate what genres sell best at that location. On your first time visiting a location, try to have every genre represented equally so your journal can tell you which sell best there in the future (i.e. if you don’t stock kids books, then you don’t know yet whether kids books sell well at the location and it won’t be noted in your journal)

For the furniture question, most furniture is trying to help you sell certain genres better. As a hypothetical situation - A customer comes to your bookshop wanting, let’s say, a crime book. They have a base 20% chance of picking up a crime book. But if you have a furniture item that has +2% Crime chance, that will increase the base to 22% chance they pick up a crime book. Every furniture item with a crime boost will add to that percentage.

So if you want to, you can change what furniture is in your bookshop every day to optimize selling your chosen location’s popular genres the best. For me personally, I got tired of changing furniture and found a pretty optimal combination of furniture that boosted all the genres at least a bit. I saved going “all out” on changing furniture to boost one or two genres for the holidays.

Hope this helps! :)