Now this may veer the thread a bit, but Michael Crichton's Eater's of the Dead may be a good place to look. I would actually recommend the movie based on that book even more so, though - 13th Warrior. I couldn't really get through Eaters of the Dead even though I liked the concept. It will also give you a bit more of a historical angle on stories like Beowulf and the look and feel of the movie is just so authentic (I did live in Norway for a few years, so I can attest to the location looking authentic... and I'm guessing on the look of the time period, but it seems pretty safe to say that too).
If you are looking at something a little less grounded in reality, as others have said, John Gardner's Grendel is wonderful. Of course, knowing the story of Beowulf helps a bit when reading Grendel.