You can buy the pack from Leanpub: https://leanpub.com/zce
There are a few other things I have published which may help you to prepare (and these don't need your credit card!). When I published this pack, I wrote a blog post giving my best advice for preparing to take the ZCE exam. Also over the last few years I have collected and actively curate a set of links to articles covering some of the topics in the ZCE syllabus, which may be helpful to anyone studying for it. I am also available as a ZCE trainer, offering both public courses and training specifically for your team, just get in touch or take a look at my upcoming courses.
In particular I've taken on aspects of community management and code quality in this project - and replaced the existing API with a newer, more robust and flexible version. Joind.in was actually the first open source project I contributed code to; seeing others take their first steps here is an aspect of the project that I find really rewarding.