Books directly related to zsh:
- From Bash to Z-Shell - A nice book that covers Bash and zsh (with an emphasis on zsh).
- Zsh - Die magische Shell - A german book on zsh; didn't read it myself, but I heard it's a useful read.
Books about shells in general:
- Shell Scripting Recipes - Not strictly a book about zsh; it covers advanced shellscripting.
- UNIX Shell Programming - Covers POSIX/SUSv3 Shell programming and related information.
- Mastering UNIX Shell Scripting - This book deals with ksh programming. zsh originally tried to work a lot like ksh, so this is useful.
- Beginning Shell Scripting - Mainly covers Bourne Shell scripting, but also discusses special features of ksh, bash and csh *ungh*
- Linux-UNIX Shells - Discusses Bourne-, Korn- and C shell, bash and tcsh. This is a book is written in German.
- Learning the Korn Shell - Well, a book about ksh. Again, as zsh originally tried to follow ksh's ways, this book could be interesting for zsh users, too.
- Classic Shell Scripting - As the name suggests, classic shell scripting is the topic here. This books tries to tell the reader about common traps, as well.
Books about UNIX tools (and therefore the shell as well):
- UNIX Power Tools - A great book about UNIX and its tools. It tells you a lot about your system and its shell with a lot of its tools.
- Practical Guide to Linux Commands, Editors and Shell Programming - How to work with Linux (and therefore UNIX) from the commandline.