Text in bold denotes books that I highly recommend.

Software Engineering

• Best Kept Secrets of Peer Code Review by Jason Cohen, Steven Teleki and Eric Brown
• Software Performance and Scalability by Henry H. Liu
• Exploring Expect by Don Libes
• Designing Data-Intensive Applications by Martin Kleppmann
• Vagrant: Up and Running by Mitchell Hashimoto
• Ansible: Up and Running by Lorin Hochstein
• sed & awk by Dale Dougherty
• Release It! by Michael T. Nygard
• Mature Optimization Handbook by Carlos Bueno
• Practical Monitoring: Effective Strategies for the Real World by Mike Julian
• The Mythical Man-Month by Fred Brooks
• Accelerate: The Science of Lean Software and DevOps: Building and Scaling High Performing Technology Organizations by Nicole Forsgren, Jez Humble, and Gene Kim
• Site Reliability Engineering

Erlang/Elixir

• Learn You Some Erlang For Great Good! by Fred Hebert
• Programming Erlang (2nd Edition) by Joe Armstrong
• Programming Elixir by Dave Thomas
• Introducing Elixir by Simon St. Laurent and J. David Eisenberg
• Stuff Goes Bad: Erlang in Anger by Fred Herbert
• Erlang and OTP in Action Paperback by Martin Logan, Eric Merritt and Richard Carlsson
• Metaprogramming Elixir by Christ McCord
• Little Riak Core Book by Mariano Guerra
• A Little Riak Book by Eric Redmond
• Designing for Scalability with Erlang/OTP by Francesco Cesarini and Steve Vinoski
• Programming Phoenix by Chris McCord, Bruce Tate, and José Valim
• PropEr Testing by Fred Herbert
• The Erlang Runtime System (The Beam Book) by Erik Stenman
• The Little Ecto Cookbook
• Genetic Algorithms in Elixir by Sean Moriarity
• Concurrent Data Processing in Elixir by Svilen Gospodinov
• Handbook of Neuroevolution Through Erlang by Gene I. Sher
• Elixir NX: The Machine Learning Toolbox by Bruce A. Tate
• Phoenix LiveView by Bruce A. Tate
• Design for Ecto: A Layered Approach to Persistence in Elixir

Linux/UNIX

• How Linux Works by Brian Ward
• The UNIX Programming Environment by Brian Kernighan and Rob Pike
• The Design of the Unix Operating System by Maurice J. Bach
• DNS and BIND by Paul Albitz and Cricket Liu

Javascript

• Javascript: The Good Parts by Douglas Crockford
• Interactive Data Visualization for the Web by Scott Murray

Bash

• Advanced Bash Scripting Guide by Mendel Cooper

Ruby

• Practical Object-Oriented Design in Ruby by Sandi Metz
• Ruby Science by Joe Ferris and Harlow Ward
• Multitenancy with Rails by Ryan Bigg
• Testing with RSpec by Aaron Sumner

PHP

• PHP and MySQL Web Development (4th Edition) by Luke Welling and Laura Thomson

Python

• Dive Into Python by Mark Pilgrim
• Foundations of Python Network Programming (2nd Edition) by Brandon Rhodes and John Goerzen
• Practical Django Projects (2nd Edition) by James Bennet

Lua

• Programming in Lua by Roberto Ierusalimschy

C/C++

Vim

• The VimL Primer by Benjamin Klein
• Modern Vim: Craft Your Development Environment with Vim 8 and Neovim by Drew Neil

Databases

• Graph Databases by Ian Robinson, Jim Webber and Emil Eifrem
• Learning Cypher by Onofrio Panzarino
• Seven Databases in Seven Weeks by Eric Redmond and Jim R. Wilson

Data Science

• Data Science from Scratch by Joel Grus
• Mining Social Media by Lam Thuy Vo
• Everyday Data Science by Andrew N Carr
• Gnuplot in Action, 2nd Edition by Philipp K. Janert

Machine Learning

• The Hundred-Page Machine Learning Book by Andriy Burkov

3D Graphics

• Digital Lighting and Rendering (2nd Edition) by Jeremy Birn
• Introducing Character Animation with Blender by Tony Mullen
• Mastering Blender by Tony Mullen

Business

• The Secret of Selling Anything by Harry Brown
• The Lean Startup by Eric Ries
• Lean Analytics: Use Data to Build a Better Startup Faster by Alistair Croll and Benjamin Yoskovitz
• Innovation and Entrepreneurship by Peter Drucker
• Getting to Yes by Roger Fisher, William L. Ury and Bruce Patton
• Platform by Michael Hyatt
• Linchpin: Are You Indispensable? by Seth Godin

Healthcare

• Principles of Health Interoperability, HL7, and SNOMED by Tim Benson
• Rockefeller Medicine Men: Medicine and Capitalism in America

Economics

• The Use of Knowledge in Society by Friedrich Hayek

Ham Radio

• The No-Nonsense Technician-Class License Study Guide by Dan Romanchik

Mathematics

• Mathematics for the Nonmathematician by Morris Kline
• Introductory Statistics by Barbara Illowsky and Susan Dean
• Think Stats (2nd Edition) by Allen B. Downey