The Python docs themselves is a good place to start: https://docs.python.org/3/reference/datamodel.html#slots

The Python wiki also has some good info about it: https://wiki.python.org/moin/UsingSlots

Apart from the official docs, this video also explains the low level data layout (in CPython) that slots introduce: https://www.youtube.com/watch?v=Iwf17zsDAnY