I'm not sure if you have looked at these yet but some medieval Rus primary sources are the Novgorod First Chronicle (http://faculty.washington.edu/dwaugh/rus/texts/MF1914.pdf Translated into English) the lavrentiev chronicle to which I cannot find an English link and the Galitsko-Volynskaya letopis for which I can't find a free translation either. If you have access to academic articles then I would suggest searching for these. I'm hoping these help you as a starting point in finding the information you are looking for.