A few years ago I read The Pragmatic Programmer. The Pragmatic Programmer may not be the GoF book, but it's almost as valuable.
Andy Hunt and Dave Thomas recommend learning a new language every year. I started with Groovy.
Being a Java developer I thought Groovy would give me the most bang-for-the-buck. After all Groovy is part of the Java spec so there is no excuse not to learn it.
I think Groovy delivered. I used to think that Groovy was worth learning for XML parsing/writing alone. XMLSlurper and MarkupBuilder are by far the best things I've used for dealing with XML.
Gradle changed my mind. Gradle is a fantastic build tool. Maven solves some problems, but it is a whole set of problems itself. Gradle can run Ant tasks so you keep Ant's functionality.
As for the one language per year thing I picked up Python next and got a Django app into production. I'm on Ruby now, but I'm behind schedule. I'll get to Scala soon...