"The Debian Administrator's Handbook"[0]. Free, comprehensive, updated. Every other book is 2/3 at best. This also includes the venerable TLDP[1], which will've perhaps recommended 15y+ ago. You can also choose the most practical approach and attempt "Linux From Scratch"[2].

[0]: https://debian-handbook.info/ [1]: https://tldp.org/index.html [2]: https://www.linuxfromscratch.org/lfs/read.html (perhaps systemd vers better if wanna learn modern Linux)

20 years ago I just practiced with the core utilities.

https://wiki.archlinux.org/title/Core_utilities

I created a large 8 char password file, then manipulated it with. sed, cut, head, awk, join, paste, sort, tr, uniq, and many others.

sounds boring but it was great fun for me.

really simple stuff:

  making the file 10 chars long
  adding digits to the end of each line
  adding a space to the begining of each line
  adding a special character to the middle of each word
  putting the word file into alphabetical order

etc etc. it got me learning how to navigate the terminal and not rely on extetrnal tools

--help is your friend