I highly recommend The Varangians of Byzantium. It's a somewhat difficult read in parts but it's just about the only good book on the Varangian Guard that's out there, to my knowledge.

It's (highly researched) fiction, but the first and last books of the graphic novel series Northlanders feature a former Varangian who returns to the Viking life. He recounts some details about his life as a Varangian, and it's a good look into the mindset and lifestyle of the people who were Varangians, even in a different context. Plus it's a great read.