Useful historical information can be hard to find. It's easy to find the big facts like when was the Viking raid on Lindisfarne, or even who did what at the Battle of Hastings.
But, for the Bard and Sigurd stories, I needed to know what are they ate in medieval northern England? Lots of steak & potatoes? Not even close. Potatoes are a New World vegetable and cows were more valuable for milk.
And clothing? Doublet and Hose? (Nope, that's more rennaissance), tunic and leggings (probably), robes (for the upper classes).
Two of the more useful sites I found were unexpected.
Would you believe out-of-copyright comic books? In particular, the Robin Hood comics published by Magazine Enterprises during the 1940s and 1950s had a text section describing life in medieval times. Robin Hood is a little out of period for Bard & Sigurd, but I pulled a few snippets from these articles.
The downside of things like the Digital Comic Museum or Comic Book Plus is that you can lose a few hours (or more) to The Spirit or other classic titles.
The other fun bit of research was two books by Paul Telegdi: Learning Berserk and Unlearning Berserk.
These are stories about the life of a norse slave in the early 11th century. Telegdi went into a lot of detail about how his characters lived, what they ate, how the portages between the North Sea and Black Sea were done, etc.
The books are good historical stories and fun reads, but the gravy for me was the level of research that went into them.
And as free samples, you can't beat the price.
My research into Constantinople has led me to French Romantic painters, Assasin's Creed video game, and articles on belly dancing.
All of this is on top of the normal sorts of research like books and history websites.
I should mention that Paul Telegdi has written other books, and they are good reads.