Hi! I'm going to read it!
And although I could read it today -- not because I'm special but because it was already published in Tolkien Studies, and I happened to buy the particular volume of TS with this tale in it, when it came out -- I think I might wait until this new book arrives. Hammond and Scull have already pointed to some general differences between Tolkien's story and the Finnish tale (Tolkien enhanced the role of the dog Musti for example). And since I've already read their commentary, hopefully I'll forget some of it by the time I read the new book.
I waited this long in order to finnish the whole Kalevala, and thus read the Kullervo verses first (I finished all the verses just recently), that is, before reading Tolkien's version in prose (with bits of poetry).
Although if I wait much longer I'll probably forget some of the finer details in the Finnish version, but if I do I can now look up the verses without feeling that I'm "spoiling" a first read.
Not that you needed or wanted to know any of this
It's arguably not "necessary" to read the entire Kalevala even if you (anyone) want to read the Kullervo verses before Tolkien's prose story. I mean, it's not like you won't get the basic story, even if it's part of a larger whole (the "whole" of the Kalevala and its construction is quite an interesting subject in itself, and for example, most people are probably reading a longer version than was originally published).
Anyway, what was the question
Of course it's not necessary that one even read the Finnish Kullervo "section" before
I just wanted to