What is the best source for cross-language description of language features? Eg: “optional types”: which langs have them and how do they work? Same with “union types” etc.

There's no single oracle for these things, unfortunately. One common citation is "Types and Programming Languages" by Benjamin Pierce (2002), but otherwise people generally just use the nomenclature of a specific language and hope cross-linguistic adventurers will figure it out. I think there's somewhat less variance with more modern languages, though, so in some ways it's less of an issue than it once was.

Rosetta code is often helpful. Not exactly for language features, necessarily: https://rosettacode.org/wiki/Monads/Maybe_monad