The movie
The Thirteenth Warrior is based on
Michael Crichton's book
Eaters of the Dead (Which incidentally, I saw in a
Barnes & Noble recently, renamed to
The Thirteenth Warrior as well.)
BUT...Crichton based Eaters of the Dead/The Thirteenth Warrior roughly on Beowulf, also referencing ancient manuscripts and writings regarding Beowulf. He has a forward talking about his interpretation of the story of King Hrothgar, Beowulf and the Grendel.
If you don't have the patience/willingness to read old english literature, read Crichton's book. It's worth it. If you do have the patience, see Beowulf on Everything or buy a copy translated by Burton Raffel.