Hey EntropicLove.
I'd recommend learning about memory management systems, and the partitioning and design of memory with respect to the kernel, OS application, and other application levels.
I'd recommend you look at the device driver framework for modern OS platforms and I would also suggest you look at multi-threaded paradigms at both the kernel and application level.
Finally, take a look at the modern data structures that the kernel uses for I/O, networking, memory management, application execution, system and application libraries, system executables, and cache management.
Since the linux kernel is fully available open source, this will give you a real repository to look into and if you can find a decent guide, then it will keep you busy for a while.