This is exactly what I am trying to establish to improve my bumpy English. My best guess is that the correct form is "Brooks's" because (1) "Brooks" is a singular noun ending with an "s" and (2) it is not a classic neither religious name. If you claim it should be "Brooks'" I am ok with this as long as you give me a sensible explanation.
There's not exactly a consensus these days on what is correct. Either is valid, but I generally prefer _Brooks' Law_ to _Brooks's Law_ since it looks more clean. Of course, Brook's Law is incorrect, as there is no "Brook"
Native English speaker, from England, and we were explicitly taught to use Brooks' rather than Brooks's.
However that didn't stop the Beatles from using "In an Octopus's Garden" as a song title. (Note that the song is about a single Octopus). I would suggest that it depends on whether you intend to explicitly repeat the 's' when speaking.
Plurals of words ending with an 's' are an occasional minefield. You sometimes hear people smugly insist that the plural of Octopus should be Octopi, only to have someone even more smugly point out that Octopus is from Greek, not Latin, and so it should be Octopodes. Meanwhile the rest of us just continue to use Octopuses....
