The Z Shell Manual

The Z Shell Manual contains a description of the shell that is complete, concise and up-to-date, although somewhat overwhelming and devoid of examples. 

Zsh is a shell designed for interactive use, although it is also a powerful scripting language. Many of the useful features of bash, ksh, and tcsh were incorporated into zsh; many original features were added. Zsh assumes basic knowledge of the standard UNIX shells; the intent is to show a reader already familiar with one of the other major shells what makes zsh more useful or more powerful.  

A free copy as a PDF may be found on SourceForge.