What only a small subset of highly specialized programmers should know about memory (2007) [pdf]
I also like to follow Tobi Lütke. The guy is the founder/CEO of Shopify (with all which that might entail), and he still tweets amazingly interesting low level coder stuff.
I concur. There's something about concurrency and multithreading that ticks your brain in a weird way. It keeps me stimulated.
My favorite is setting up a scenario that would normally be considered as a disasterous concurrency failure. Like intentionally messing up a `volatile` state. You can't ever have those in prod, of course, but seeing them in a controlled, _expected_ way tickles my brain in a very perverse way.