It's a "work in progress" in the sense that the professors aren't quite completely satisfied with it. It's still very readable, which is even more impressive when you consider the complexity of the topics being covered.
If you're interested, I'd recommend at least glancing at the 15-210 schedule[1] so you can see the order that the chapters are covered in the class.
As a person who is finally getting time to read papers in this field, I came across a Stack Overflow post [1] about newer functional data structures recently when looking for papers to put on my kindle for a long trip. The top answer to it has a tonne of links.
Tangentially related, but I am currently reading Pearls of Functional Algorithm Design [2] - It is fascinatingly well written (though it isn't strictly about data structures only).
I plan to read Fun of Programming [3] next which has a chapter on binary heap trees by Okasaki but the rest of the topics aren't quite about data structures.
No, none. That was the only publication on (necessarily) functional datastructures in the past 100 years. Maybe Turing and Church, but that's really it. Turing was especially explicit about keeping state (the notation) explicit.
Do you mean recommendations? I have none, but SICP, which you probably know. I liked the first chapter so far.
Also, see wiki (https://en.wikipedia.org/wiki/Purely_functional#External_lin...), and citations and similar articles for Osaki on google scholar. Personaly, I find the underlying theory about logics more interesting to begin with. ML was a script language for proof automation, in the bigger picture.
To hijack your thread: Seeing that OOP should be possible without side-effects, I find the distinction arbitrary. The basic structures are lists. Objects are often implemented with tables.