Zabbix community
When you have finished the recipes in this book, you should have a good understanding of how Zabbix works, but you will probably also have many more questions on how to monitor certain things in your environment.
I encourage you to join the Zabbix community. Zabbix provides a page where all information is gathered from the community, http://www.zabbix.com/community.php.
A lot of information can be found on the Zabbix forums where people have posted questions and received answers for their problems, http://www.zabbix.com/forum.
If you would like to help Zabbix improve or write some hacks of your own then you could download the source code, http://sourceforge.net/projects/zabbix/ or http://www.zabbix.com/developers.php.
If you need some quick support, your best bet is probably the IRC #zabbix
channel on Freenode, where lots of people check in every day, including most of the Zabbix developers: http://webchat.freenode.net/?channels=#zabbix.
Most importantly, don't forget to...