Although this is not a gentle introduction, I heavily recommend the O'Reilly press book "JavaScript: The Definitive Guide" by David Flanagan. When you are dealing with JavaScript -- and learning it -- there are two different types of questions you face. One is more or less what you asked: how do I figure out how to do something. But the other you will hit all the time as well: What the heck is this piece of JavaScript I'm looking at supposed to do?

The O'Reilly book is probably the key tool to answering that second question. And, as you learn JavaScript, you're going to want to look at how sites have done things you like. An intro-to-JavaScript book is going to skip over some of the more obscure or tricky things you can do; you need a solid reference to get you through.

You will never outgrow this book -- you'll just keep buying new editions of it as it keeps up with JavaScript developments (It's on its 6th edition!).