It's the closest thing we have to a new Ruby Hacking Guide. It's a somewhat gentler, more approachable treatment of the subject matter, but no less awesome.
Shaughnessy also has a number of blog posts on his site that dive into the same kind of details covered in his book (and RHG).
Ruby changed my life, it changed my mindset.
Has anything fundamental changed in how Ruby manages objects or does garbage collection?
Garbage collection has changed from regular mark-and-sweep to lazy sweep to using bitmap marking.
Unless there is something specific about the MRI implementation and class variables?
"A class variable is shared across all instances of a class, which includes all subclasses". Can be surprising when using inheritance.
It is the kind of bug that can be very long to figure out, since when you make this mistake, it's very likely because you forgot to think about this behavior.