The Pan Docs, at http://problemkaputt.de/pandocs.htm , have pretty much all the bare metal information you could want. After you define macros for everything, it shouldn't be much lower level than most libraries.
I actually have a bit of half-finished GB code that does sprites and backgrounds, without using libraries. If anyone cares for it, I can put it up on Github later.
In the interest of actually getting this out tonight, I have a full 7z archive of my development environment up at https://juliangoldsmith.com/rgbds.7z .
I'll most likely clean up the sources and put them up on Github later.
I actually have a bit of half-finished GB code that does sprites and backgrounds, without using libraries. If anyone cares for it, I can put it up on Github later.