This is how I learned unix shell back around 2000ish. Aside of giving an excellent insight into zsh, it also gives many good hints and notes about unix shells in general.
The first two or three chapters of that guide are pretty good, but then it gets bogged down in obscure minutia to such a degree that you start to feel almost like you're just reading the zsh man pages again.
The guide is in some serious need of a good editor to make it more concise and better organized. It also needs many more simple, practical examples. Unfortunately, it hasn't been updated since 2002, which also makes it a bit obsolete, since zsh development has been very active since then and zsh is now on to a new major version.
Overall, the guide is a nice try, but zsh needs more and better documentation if its not going to overwhelm all but the most dedicated users.
This is how I learned unix shell back around 2000ish. Aside of giving an excellent insight into zsh, it also gives many good hints and notes about unix shells in general.