JQuery is a very popular JavaScript library. From the web site:
jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.
For business software developers JQuery is a bit of an unfortunate name as it often conjures up visions of querying SQL databases. Querying is central to JQuery's usefulness, but you're not querying a database, you're querying the HTML page itself which is typically made up of many elements. JQuery makes it easy to locate these elements and do things with them (much the way CSS locators work, except that with JQuery you're applying the vast power of JavaScript to the page).
JQuery has a plugin architecture which makes it easily extended; as a result there is a rich ecosystem of JQuery plugins.
John Griffiths recently started a thread in the softvelocity.clarion.third_party newsgroup titled "jQuery book recommendation". The following books were suggested:
jQuery Novice to Ninja (recommended by John Griffiths)