The free PDF of William Shotts' book, hosted on SourceForge as linked from linuxcommand.org. Read it start to finish or dip into chapters.
Read the full PDF on sourceforge.netSource: The Linux Command Line (PDF, 25.12A) by William Shotts. Licence: CC BY-NC-ND 3.0.
A multi-part online tutorial covering navigation, viewing and manipulating files, redirection, expansion, permissions and job control. It opens with a short example of why command-line skills are worth learning.
A free 596-page book that teaches the shell and shell scripting from scratch and then moves on to common programs and advanced topics. The page links the PDF download.
A gentle introduction to the terminal: opening it, moving around the file system, creating and deleting files, pipes, man pages and safe use of sudo. The page says it has been replaced by a newer version and targets Ubuntu 18.04.