Other relevant authoritative teaching on these subjects can be found on the same website, including the document of the Second Vatican Council entitled Unitas Redintegratio and the encyclical commemorating the apostolate of SS. Cyril and Methodios Slavorum Apostoli.
There is also the Pope's congratulatory apostolic letter Euntes in Mundum on the thousandth anniversary of the evangelization of the Kievan Rus - but unless you know Latin or Italian you won't be able to read it.