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