How do I found out if a book or author is really a best-seller?

Cafe Society
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Spice Weasel is named after a Futurama reference. Spice Weasel likes Futurama quotes. Spice Weasel mentioned wanting to write SF romance. I’m 99% sure Spice Weasel got the joke I was saying to Spice Weasel. It was neither random nor off-subject.