I'm at a loss, other than Astrix (in which the vikings only appear once or twice through the series).
Of course, they appear in many of the books involving Norse mythology, and that may be a place to start as well. Thor, Loki, and many others are mentioned, are characters, and good old Asgard (I think that's the name) serves as a backdrop for portions of several stories out there (and many many movies, comic books, graphic novels, etc)...though I can't think of any titles off hand.
The vikings appear as an enemy, if memory serves, in Jack Whyte's Arthurian Skystone/Camolud series - though I'm not sure you'll find these in e-book format other than the one or two that have been sited on the darknet, and as a Canadian author, they might be hard to find wherever you live. It's been a while since I've read the series (in pBook when each was first released), so I could be mistaken on the vikings/Norse being in the story at all though, and if they are, I think it was only in a couple books.
You got me curious, and I did stumble over these links:
http://bookviewstest.blogspot.com/20...g-fiction.html
http://www.librarything.com/topic/18800
http://www.vikinganswerlady.com/Viki...edNovels.shtml
http://www.fiveeightforums.com/vikin...40/index.html?
http://www.historicalnovels.info/Med...andinavia.html
http://www.historicalnovels.info/Medieval.html