I'm looking to buy an introductory book and am on a tight budget. There are some good prices for second hand books on Amazon, but I don't want to make a mistake like buying a C# v2 book when v4 is out.
Wikipedia sheds no light, even in the History section of the JS page. For the statement "buy a book published after xxxx and you will be ok?", can someone tell me the value of xxxx?
Anything based on ECMAScript 5th edition will make you future proof for at least the next few years. This is the specification most modern browsers are targeting.
ECMA-262 5th Edition ECMAScript Specification
However, reading some of the great, existing javascript books will definitely do you no harm. Read "JavaScript: The Good Parts" by Douglas Crockford and "JavaScript: The Definitive Guide" by David Flanagan and you will be set since -- after all -- the language has not changed that much (though the improvements are definitely welcomed!) with regards to the fundamentals.
If it comes to it and you want to learn more about some of the goodies introduced in the latest edition, there's nothing you won't be able to find from reading one of great JavaScript blogs out there or just reading the specification itself. And both of these are free!
Here's a nice Google Tech Talk discussing some of the changes to JavaScript in the 5th edition: http://www.youtube.com/watch?v=Kq4FpMe6cRs (slides in PDF).
Officially, JavaScript 1.8, or ECMAScript 5. (Wikipedia: JavaScript#Versions)
However, don't worry about versions much when it comes to books. The practical definition of the language ("whatever works in most browsers") has not changed much: read reviews, and rather buy an old, good book than a new junk publication.
Are you sure that you want an actual paper book?
Check out http://eloquentjavascript.net/ which is a mix of online book and testbed. It will let you run all the example code and experiment with it as you learn.