Reddit mentions of FreeBSD Device Drivers: A Guide for the Intrepid

I can't recommend that book enough. It will give you a great overview over the services the kernel provides, design decisions and data structures.

In addition to that, these resources might also be of interest to you:

• Stevens, TCP/IP Illustrated Vol2. This is the actual 4.4BSD Lite tcp/ip stack source code; commented and explained. http://www.amazon.de/TCP-IP-Illustrated-II-Implementation/dp/020163354X/
  
• Kong, FreeBSD Device Drivers. Kernel module and device driver programming. http://www.amazon.de/FreeBSD-Device-Drivers-Jospeh-Kong/dp/1593272049/
  
• Kong, Designing BSD Rootkits. More kernel module programming :) http://www.amazon.de/Designing-BSD-Rootkits-Introduction-Hacking/dp/1593271425/
  
• https://github.com/teachbsd/course hands on course material, understanding OS internals via DTrace and FreeBSD

I recommend *BSDs as learning command line is easier there than Linux due to centralized documentation and consistent tools that doesn't change as much as Linux user land. For desktop use, I think all Free Unix-like machines suck regardless of DE. Read this: https://www.freebsd.org/doc/en/articles/explaining-bsd/article.html

• FreeBSD: Focus on performance and robust/secure storage.
  
• NetBSD: Aims to run on any architecture, focus on portability.
  
• OpenBSD: Clean coding and secure practices.
  
  The difference is BSDs have separate userspace at /usr/local/<here>. Unlike Linux, BSDs are developed as a single OS, so it's not something like GNU+Linux+musl+busybox, it's just FreeBSD/OpenBSD/NetBSD. Remember, don't confuse these BSDs as Linux distros. Each of them are different OS, with different kernel. Also due to consistent nature you can't replace core system parts like NetworkManager, dhcp etc on runtime via package manager. If you install something later, it will be installed into /usr/local/bin/<here>... why.. because it's not part of FreeBSD where core system parts lies on /bin/, /usr/bin, /usr/sbin etc. Due to this design, your system remains more failsafe, less vulnerable to crash etc. I think you should start with FreeBSD Auto ZFS install. If you want to run FreeBSD on decent modern hardware, try TrueOS. But do not expect anything like Linux. Both are completely different. Also the community is full of technical people. There is no DE/distro war on BSD land (Because there is no such things there, btw still some Linux newcomers make noise sometimes). Nobody asks why X sucks.. etc :D Most of the BSD people are very experienced system programmers and some of them are also university professors. I still recommend using Linux at desktop if you are kinda Free Software guy. I am not, however I am fine with something as long as it is Open Source (may/mayn't be GPL etc). I also use tons of proprietary applications and I absolutely love them. Try FreeBSD on a VM, not on real hardware for now.
  
  Check this out, when this guy moved to BSD after being a long time Linux user: https://runbsd.info/people/fr0xk.html
  You will also enjoy this sort of stuff: https://www.freebsd.org/doc/en_US.ISO8859-1/articles/new-users/index.html
  
  What to learn:
  
  
• Bourne shell (not bash, but sh)
  
• cat, less, grep, sed, awk and vi
  
• Unix group permission handling and user managements
  
• Init and service management, what is init system, and how boot process works in BSDs/Linux etc
  
• File system, what is data? What is journaling. How do you ensure data integrity etc.
  
• Modern filesystems (btrfs, ZFS), and logial volume management (LVM). What are advantages/disadvantages of them.
  
• Stack, Address Space Randomization, Process randomization, userspace vs kernel space.
  
• Read books: https://www.amazon.in/FreeBSD-Device-Drivers-Guide-Intrepid/dp/1593272049 (If you are still reading this) xD
  
  I was a Linux noobs for 10+ years, until I started to study computer science as hobby (I am non-geek, non tech guy, and barely knew how to turn the smartTV on)