The pure bash bible is such an impressive resource of bash tips and tricks catalogued in a very readable way, with excellent examples.
It shows how much you can do with bash built-in methods. Even though I am a bit split on how much these should be used, as most of these will make your scripts bash-only.
On the other hand, these could be a life saver if you are working with bash without a full unix environment. For example when working in some embedded low storage system, or when working with git-bash in windows. The optimization of not forking a new process for no reason, is also a nice bonus.