Since you do not mention if you're already a pro with OS internals and implementations, I will assume, maybe incorrectly, that you're not.

So in that spirit, try the XinuOS. It has the complete source code for a minimal, clean and practical OS. Search for the book explaining it inside out.

xv6, a re-implementation of Dennis Ritchie's and Ken Thompson's Unix Version 6 (v6) also comes to mind.

And C. Don't forget to sharpen your C skills.

Read the Linux programming interface by Michael Kerrisk. It will tell you what Linux should be doing for you ( the api basically and what subsystems exist). Read lwn, kernel newbies to get a feel of what’s happening in the kernel.

Get knowledge of operating systems ( I like the minix book by tanenbaum ). Learn about Linux internals ( I like linux kernel development by Robert love)

Then, pick bits you’re interested in and jump !

Linux Kernel Development by Robert Love. It's spectacularly out of date but is still a good starting point. And Elixir is very helpful for browsing the source code and comparing chnages across kernel version: https://elixir.bootlin.com/linux/latest/source

Linux Core Kernel Commentary: Guide to Insider's Knowledge on the Core Kernel of the Linux Code

https://www.amazon.com/Linux-Core-Kernel-Commentary-Knowledg...

Understanding the Linux Kernel, Third Edition 3rd Edition

https://www.amazon.com/Understanding-Linux-Kernel-Third-Dani...

Without more details I assume you just want the source code and documentation. One could either clone the repositories [1] or just browse the CDN access to the docs and kernels. [2] The latest kernel is in the v6.x directory.

[1] - https://github.com/torvalds/linux

[2] - https://cdn.kernel.org/pub/linux/kernel/

Have you considered ''Linux From Scratch'' as an element of your research?

https://www.linuxfromscratch.org/

Building a Linux kernel and system with your own specific configuration tweaks may reveal things that the best of books, manuals, courses, etc. might not cover.

Something I've done a couple times is trace the execution of a system call by reading the code. LXR is really helpful.

This older comment thread may be useful: https://news.ycombinator.com/item?id=19225268

Understanding the Linux Kernel, Third Edition is now horribly out of date, but it's a good place to start if you've never read it before.

find something you don't like about linux and try to track down where that thing is to fix it