A few years ago (yes really), I wrote a O'Reilly shortcut on how to build Zimlet's for Zimbra. The market conditions changed after I started to write it, and O'Reilly recently informed me that they were giving up on publishing some shortcuts including mine, even though Andy Oram, editor extraordinaire and generally good guy had put a lot of effort into hammering my document into something useful.
So I am happy to announce that I got their agreement to go ahead and publish it to the Zimbra community. I am going to shoot to have one section a week published, and the first section - on general Zimbra architecture is now up.
See here for more details:
Zimlet Development - Zimbra :: Wiki
Please feel free to add feedback below, it will be a couple of sections before I start diving into code, but please let me know if there are issues with any of the code base!