Today I read an article that is a great introduction to what's going on around the print edges of those old manuscripts. You can find it here. And if you're already a fan, like me, perhaps you'll find more to interest you in the British Library's blog -- it's a fun place to browse around on a rainy afternoon.
The illuminated manuscript I see most commonly referenced is the Luttrell Psalter. Its margins are full of everyday medieval life, along with wicked beasts and battling animals. There's a movie that's been made by reenactors that really brings the Luttrell margin artwork to life. I think I've linked it here before, but it's so good I'll put it up again. Enjoy!