I'd say the ideal situation is to borrow a book you want to read from your local library. Second best would be to buy the book, assuming you plan on rereading it often, or want to give or lend it to someone you know will like it.
The dumbest thing would be to buy a book not based on its content, but on the color of its spine so that you can turn your bookshelf into a rainbow.This Etsy seller enables that precise practice, by selling "decorative books" that have different prices based on their color.
Incredibly, these are sold by the linear foot.
To be clear, these are real books; the seller has just sorted them by color and come up with different dollar values for them based on popularity/scarcity.
While I think this practice is dumb, I have to concede that the seller is actually the smart one: They've made 6,122 sales and have a perfect 5-star rating.