Mark pilgrim wrote the book “Dive into greasemonkey” which is a cool book to start with. If you want to read it online, just visit this url
For those who are interested to know what is greasemonkey, read this intro taken from that book.
reasemonkey is a Firefox extension that allows you to write scripts that alter the web pages you visit. You can use it to make a web site more readable or more usable. You can fix rendering bugs that the site owner can’t be bothered to fix themselves. You can alter pages so they work better with assistive technologies that speak a web page out loud or convert it to Braille. You can even automatically retrieve data from other sites to make two sites more interconnected.