I like the format of the series. As a child I would have *loved* to have travelled via a treehouse to distant times and places. What child wouldn't want to meet characters from somewhere else? I like the idea of fetching books- I love books so the task would be appealing to me.
From my reading of Viking raids, it is very likely that raids took place as described here, against the Irish monks. Vikings were in search of stored foods, wealth and whatever else they could grab on a quick morning attack. There would have been many ships and a fast assault as portrayed here. Many aspects of the raids were left out, and that is just fine for the young school-age reader.
These are chapter books that can be easily read by an advanced first grader over several days, or read as bedtime stories by parents over one or two evenings. There are some illustrations to augment the storyline, but the main focus of the book is the text.