Update, a note from the author:

This book is badly out of date and will probably never be updated. You can find more up-to-date information on the Greasemonkey wiki.

Another update:

You can download the original book here.

Terms and Conditions:

Mark Pilgrim wrote:This book, its sample code, and its supplementary videos are free software. You can redistribute them and/or modify them under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License or (at your option) any later version.

Book Excerpts:

Greasemonkey is a Firefox extension that allows someone to write scripts that alter the web pages he/she visit. It can be used to make a web site more readable or more usable. It can fix rendering bugs that the site owner can't be bothered to fix themselves. It can alter pages so they work better with assistive technologies that speak a web page out loud or convert it to Braille. It can even automatically retrieve data from other sites to make two sites more interconnected.

Greasemonkey by itself does none of these things. In fact, after its installation, no immediate changes can be noticed ... until you start installing what are called "user scripts". A user script is just a chunk of Javascript code, with some additional information that tells Greasemonkey where and when it should be run. Each user script can target a specific page, a specific site, or a group of sites. A user script can do anything Javascript can do. In fact, it can do even more than that, because Greasemonkey provides special functions that are only available to user scripts.

This book grew out of discussions on the Greasemonkey mailing list, and out of author's own experience in writing user scripts. It documents the most useful common patterns found in the forum, chunks of reusable code that solved specific problems that cropped up again and again, plus explanations of author's own coding decisions.