jQuery is a JavaScript library designed to make designers and front-end developers' lives easier. According to W3Techs, jQuery is the most popular JavaScript library by far, and is in use by 33.1% of all websites.

If you're a designer or front-end developer, it's definitely something worth knowing. Here are some resources to help you learn and master jQuery.

jQuery Fundamentals

jQuery Fundamentals by Rebecca Murphey is a free e-book available in HTML. It even includes an overview of JavaScript as its first chapter, to help developers get up to speed.

It also contains contributions by James Padolsey, Paul Irish, and others also contributed to the book. You can get involved with the book yourself on GitHub.

jQuery: Novice to Ninja

jQuery: Novice to Ninja by Earle Castledine and Craig Sharkie is a book published by SitePoint. If you're willing to hand over your e-mail address, you can get a PDF with some sample chapters for free. However, much of the free material can be found without registration in SitePoint's jQuery section. The code archive for the book is also free.

Essential JavaScript & jQuery Design Patterns

Essential JavaScript & jQuery Design Patterns by Addy Osmani is a free e-book of design patterns for those who already know jQuery but want to take their skills to the next level. "There's some really fantastic literature available out there on design patterns in JavaScript (Stoyan Stefanov's excellent 'JavaScript patterns' book comes to mind) but a lot of the JS-centric material requires purchased literature," explained Osmani on the book's Web page. "These books are definitely worth buying, but I personally feel that until a developer is interested enough in learning about patterns to make the leap to invest in these, the majority of material they have accessible online doesn't focus enough on how to apply design patterns to JavaScript or jQuery."

The book is available in PDF, ePub and HTML.

Official jQuery Documentation

The official documentation for jQuery is well regarded, so if you'd rather just skip the books and just dive in, it's a good place to start.

Bonus

If you want even more jQuery stuff, check out Speckyboy's 65 Excellent jQuery Resources post.