The Linux Command Line: One of the best Linux books I have ever read. Written by William Shotts.
The Debian Administrator's Handbook: This book teaches the essentials to anyone who wants to become an effective and independent Debian GNU/Linux administrator. Written by Raphaël Hertzog and Roland Mas.
The Slack Book: Official Slackware book created by David Cantrell, Logan Johnson & Chris Lumens. Version 2.0 is a draft that is in development.
A byte of Python: It serves as a tutorial or guide to the Python language for a beginner audience; written by Swaroop C H. License Creative Commons Attribution-ShareAlike 4.0
Think Python V3: Think Python is an introduction to Python for people who have never programmed before – or for people who have tried and had a hard time; written by Allen B. Downey. License Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
The Hitchhiker’s Guide to Python!: This handcrafted guide exists to provide both novice and expert Python developers a best practice handbook for the installation, configuration, and usage of Python on a daily basis. Project started by Kenneth Reitz. License Creative Commons Attribution-ShareAlike 3.0
Think Java V2: Think Java is an introduction to computer science and programming intended for readers with little or no experience; written by Allen B. Downey and Chris Mayfield. License Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
Introduction to Bash Scripting: Introduction to Bash scripting that will help you learn the basics and start writing awesome Bash scripts to automate your daily SysOps, DevOps, and Dev tasks written by Bobby Iliev. License MIT License
Explainshell: Write down a command-line to see the help text that matches each argument.
shellcheck: A shell script static analysis tool, finds bugs in your shell scripts. You
can cabal, apt, dnf, pkg or brew install it locally right now.