The Roman Jakobson-Eli Fischer-Jørgensens correspondence is published in its entirety by The Royal Danish Academy of Sciences and Letters in one of their series. It will be available online later on. The correspondence has been edited and annotated by Viggo Bank Jensen and Giuseppe D'Ottavi. Additionally, the correspondance is introduced and contextualized by Viggo Bank Jensen. You may find further reading on the presentation of the book here.
Viggo Bank Jensen describes the genesis and content of the book "From the early years of Phonology" in Politiken's "Udkommet". Click on the following link: https://politiken.dk/kultur/boger/art7809697/Korrespondance-mellem-to-af-historiens-største-lingvister-er-bevaret-fra-start-til-slut?shareToken=aFhVGwAAQVyQ