Good question! The best primary source texts are a pair of passages known as Ohthere's Voyage (or Ottar's voyage) and Wulfstan's Voyage. Ohthere was a Norwegian aristocrat, likely from Lofoten in northern Norway, who traveled between Norway, England, and Denmark in his trade. Wulfstan had an English name but was comfortable plying the waters between Denmark and various places in the Baltic. Both travelers visited the court of Alfred the Great in Wessex, who had descriptions of their travels inserted into a Latin geography book he was having translated.

You can read a good translation of their accounts here. Their travels inspired a set of conferences and two excellent scholarly volumes: Ohthere's Voyages and Wulfstan's Voyage.

I'd also strongly recommend a pair of articles based on archaeology: Søren Sindbæk's Small World of the Vikings [also here] and Steven Ashby et al.'s Urban Networks and Arctic Outlands. The second article isn't available for free, but you should be able to find it through your local or university library website or request it from your interlibrary loan service.

This is an exciting and vigorous area of research. If you're looking to dig into the scholarship a bit, the secondary works cited about should provide a few good entry points.