Lesser known is what to do if the files don't already exist. This command will copy the filename to filename.bak. You won't have any problems after reading Chapter 17, which has 24 pages devoted to find. File permissions and processes are also covered in depth in their own chapters. Even wild cards get their own chapter.
Whole chapters are also devoted to printing, terminal and serial line settings.
The important thing to keep in mind is no matter what level of UNIX or Linux expertise you have now, you can learn much more. Here's a question for you. Since so much of the book is devoted to shells, this is bothersome. Still, since zsh is largely a superset of the other shells, almost all of the tips are still helpful. Besides, in my opinion, all you need to know about sed and awk these days is that you use s2p and a2p respectively to translate your code to Perl.