Good, quick introduction to Go. The first half tries to be an introduction to programming in general but the second half makes too many assumptions to serve that purpose. It definitely helped me translating idioms from other languages into Go-speak. It's hard to regret the time spent reading such a short book but I feel it was worth it regardless.
It is the “go to” Ruby book for a reason. If you want to learn Ruby (why wouldn't you?) you pretty much HAVE to read it. A number for things that I didn't quite understand earlier, have just “clicked” when reading this book. I definitely feel like knowledge I got from it was worth the time spend reading this book.
If you're a complete complete Ruby novice (like me) you will want to read straight through the first 3 parts. Part 4 is a dry Ruby Library Reference, so you may want to leave that out until you need something specific from it.
If you know a bit about Ruby and programming in general you may simply refer to specific chapters for help in understanding specific topics. The chapter on metaprogramming is a doozy. :-)