1. Systems Operations is first and foremost about understanding systems, in all of their complexity, which means understanding the internals of your OS primarily.
2. Performance and networking, in particular, are super important areas to focus on understanding when it comes to learning the topic to help with software development.
3. A lot of it is about understanding concepts in abstract and being able to extrapolate to other situations and apply these concepts, so there's actually quite a lot of useful information that can be learned on one OS and still applied to another OS (or on one game engine and applied to another, et al).
Here's a few books I think are worth reading, not in any particular order of prevalence, but loosely categorized
High Performance MySQL: https://www.amazon.com/gp/product/1449314287/
SQL Queries for Mere Mortals: https://www.amazon.com/gp/product/0321992474/
The Art of SQL: https://www.amazon.com/gp/product/0596008945/
TCP/IP Illustrated: https://www.amazon.com/exec/obidos/ISBN=0201633469/wrichards... (updates on author's site at http://www.kohala.com/start/tcpipiv1.html)
The TCP/IP Guide: https://www.amazon.com/TCP-Guide-Comprehensive-Illustrated-P...
UNIX Network Programming: https://www.amazon.com/dp/0131411551
Beej's Guide to Network Programming: http://beej.us/guide/bgnet/
Operating Systems Concepts: https://www.amazon.com/Operating-System-Concepts-Abraham-Sil... (various editions, I have the 7th edition... I recommend you find the latest)
Modern Operating Systems: https://www.amazon.com/Modern-Operating-Systems-Andrew-Tanen... (the "Tanenbaum Book")
Operating Systems Design and Implementation: https://www.amazon.com/Operating-Systems-Design-Implementat-... (the other one, the "MINIX Book")
Part 1: https://www.amazon.com/Windows-Internals-Part-architecture-m...
Part 2: https://www.amazon.com/Windows-Internals-Part-2-7th/dp/01354... (I had the pleasure of being taught from this book by Mark Russinovich and David Solomon at a previous employer, was an amazing class and these books are incredible resources even applied outside of Windows, we used 5th edition, I linked 7th, which has the 2nd part pending publication).
Part 1: https://www.amazon.com/MacOS-iOS-Internals-User-Mode/dp/0991...
Part 2: https://www.amazon.com/MacOS-iOS-Internals-II-Kernel/dp/0991...
Part 3: https://www.amazon.com/MacOS-iOS-Internals-III-Insecurity/dp...
Linux Kernel Programming:
Part 1: https://www.amazon.com/Linux-Kernel-Development-Cookbook-pro...
Part 2: https://www.amazon.com/Linux-Kernel-Programming-Part-Synchro...
The Linux Programming Interface: https://www.amazon.com/Linux-Programming-Interface-System-Ha...
General Systems Administration:
Essential Systems Administration: https://www.amazon.com/gp/product/0596003439/
UNIX and Linux Systems Administration Handbook: https://www.amazon.com/UNIX-Linux-System-Administration-Hand...
The Linux Command Line and Shell Scripting Bible: https://www.amazon.com/Linux-Command-Shell-Scripting-Bible/d...
UNIX Shell Programming: https://www.amazon.com/Unix-Shell-Programming-Stephen-Kochan...
BASH Hackers Wiki: https://wiki.bash-hackers.org/
TLDP Advanced BASH Scripting Guide: https://tldp.org/LDP/abs/html/
The Debian Administrator's Handbook: https://debian-handbook.info/browse/stable/
TLDP Linux System Administrator's Guide: https://tldp.org/LDP/sag/html/index.html
Performance & Benchmarking:
Systems Performance: https://www.amazon.com/Systems-Performance-Brendan-Gregg-dp-... (this is Brendan Gregg's book where you learn about the magic of dtrace)
BPF Performance Tools: https://www.amazon.com/Performance-Tools-Addison-Wesley-Prof... (the newer Brendan Gregg book about BPF, stellar)
The Art of Computer Systems Performance Analysis: https://www.cse.wustl.edu/~jain/books/perfbook.htm (no longer available from Amazon, but is available direct from publisher. This is basically the one book you should read about creating and structuring benchmarks or performance tests)
I guess that's a "reading list", but this is just a small part of what you need to know to excel in systems operations.
I would say for the typical software developer writing web applications, the most important thing to know is how databases work and how networking works, since these are going to be the primary items affecting your application performance. But there's obviously topics not included in this list that are also worth understanding, such as browser/DOM internals, how caching and CDNs work, and web-specific optimizations that can be achievable with HTTP/2 or QUIC.
For the average software developer writing desktop applications, I'd say make sure you /really/ understand OS internals... at the base everything you do on a computer system is based on what the OS provides to you. Even though you are abstracted (possibly many layers) away from this, being able to peel back the layers and understand what's /really/ happening is essential to writing high-quality application code that is performant and secure, as well as making you a champ at debugging issues.
If you're trying to get into systems operations as a field, this is just a brush over the top surface and there's a lot deeper diving required.
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