While Greek Mythology has a vast pantheon, Norse Mythology tends to revolve around Odin, Thor, and Loki with an emphasis on tall tales, reminiscent of Paul Bunyon. The stories are full of huge appetites, powerful weapons, and supernatural ship and animals.
My biggest surprise was Loki. Loki is a shapeshifter and a trickster. I thought him to be similar to Coyote in the mythology of the American Southwest, and Fox is many other traditions. However, I discovered Loki to be much darker. It starts with Thor. When something goes wrong, his first thought is that it's Loki's fault. However, Loki is more sinister than just stealing Thor's hammer. Loki causes so much death and destruction that the gods ultimately punish him with eternal bondage and torture.
Norse Mythology by Neil Gaiman makes a nice companion, as Edith Hamilton's classic emphasizes Greek and Roman mythology with the briefest coverage of Norse mythology.