From the website:
The Viking Society for Northern Research is making virtually all its publications (and some other related items) from inception in 1895 to the present freely available on this website, though recent titles may not be released until five years from the date of publication.
These digital versions are not intended to replace our printed publications, and titles currently in print will remain available to buy in book form as long as there is a demand for them (the list can be seen at http://www.le.ac.uk/ee/viking/index.html ). The digital versions are intended to make the range of our publications known to a wider public, and may be used for reference purposes, to evaluate books for purchase or for university courses and for private study. The copyright belongs either to the authors or to the Viking Society, as stated at the beginning of each work, and permission must be obtained from the Society to use downloaded versions either in whole or in part for any other purpose.