If it is literally only one book then I would go for The well-grounded Rubyist by David A. Black as a general purpose book on Ruby. There’s lots of code samples and good coverage of the language.
If you are prepared to have a few books thn…
Eloquent Ruby by Russ Olsen is great for delving into more common uses of ruby “in the wild”.
Why’s Poignant Guide is nice. It is a classic and its quirky style sort of embeds some of the syntax and conventions a little more easily than something more dry. It’s also free to read so it is a nice book if you are getting your feet wet and not ready to invest in your own personal library yet.
I see POODR more as a adjunct to other books on ruby. It’s great for applying OO principles to the specific design of ruby as a language and I think an essential read once you have a little ruby under your belt, but I don’t think it aims to teach ruby per se.