There's a really good book about the design of the slot machines, called Addiction by Design. It talks about the different UI and design elements that go into keeping the gambler engaged and why they keep spending more money. It's a really amazing and enlightening book to read if you're interested in learning more about the mechanics of the slot machines and the designs of the casino.

https://www.amazon.com/Addiction-Design-Machine-Gambling-Veg...