The Norse and the Celtic people (Welsh, Scotts, Anglo Saxons, etc) shared quite a lot, even in their mythology. Although the names are different, many of the stories are the same. So it is safe to say that they were all apart of the same belief and practice at some point, or that they just shared a whole lot in the process of travel, trade, and war. Either way, it's good to look through all of them for clues to one another. Unfortunately, the best "ancient" stuff you will find are old rocks with carvings and no instruction manual. For all you know, they were directions to the next pub, and weren't magick at all. But there is quite a lot to be gathered from the past 1500 years or so. You'll need to reverse any Christianization and it may not be 100% "authentic" to its older pagan equivalent, but it will hold the same "memory" of the many times it was used in the past. Remember, this was passed down through oral tradition, so there are hundreds if not thousands of variations throughout time and yours will just be one more added to the lot. If you look at songs of those times, it's the same way. You can find 100 different variations of the same song, but it's still the same song.
Where I have found my most "ancient" information is from academic level history, sociology, and archaeology not "magick" books.