I recently read Ionel Cristian Mărieș' Understanding Python metaclasses (via Planet Python), which is a great but deeply technical explanation of what is going on with them in Python 3. To give you the flavour, Ionel goes right down to the CPython interpreter source code to explain some aspects of attribute lookup. If nothing else, this is probably the most thorough documentation I've ever seen of the steps and order in Python's attribute lookups. There are even very useful decision tree diagrams. I'll probably be using this as a reference for some time, and if you're interested in this stuff I highly recommend reading it.

I'm personally biased, of course, so I prefer my own series on using and then understanding Python metaclasses. Ionel has a much more thorough explanation of the deep technical details (and it's for Python 3, where mine is for Python 2), but I think it would have lacked context and made my eyes glaze over had I read it before I wrote my series and wound up with my own understanding of metaclasses. But Ionel's writeup is a great reference that's more thorough than, for example, my writeup on attribute lookup order.

(But the curse (and blessing) of writing the entries myself is that I can no longer look at metaclass explanations with a normal person's eyes; I simply know too much and that influences me even if I try to adopt a theoretical outsider view.)

I do disagree with Ionel on one aspect, which is that I don't consider general callable objects to be real metaclasses. General callable objects can only hook __new__ in order to mutate the class being created; true metaclasses do much more and work through what is a fundamentally different mechanism. But this is really a Python documentation issue, since the official documentation is the original source of this story and I can hardly blame people for repeating it or taking it at its word.

PS: I continue to be surprised that Python lacks official documentation of its attribute lookup order. Yes, I know, the Python language specification is not actually a specification, it's an informal description.