There's a lot more to Happy Birthday, Monster! than just monsters. Sure, there is a mummy, a skeleton, a vampire, and a ghost involved, but there's an alien and a robot too. That is not a problem. Diversity is great, especially when dealing with guys and gals like these.
This bunch is just looking to have a good time at their friend Doris' (a lizard creature of sorts) birthday. Devilish Ben is throwing the bash, and early on we see him brushing and flossing his teeth... and then brushing and flossing his horns.
The fun of Scott Beck's book is seeing how each of these very different characters interact with each other. The book explores what happens when the ghost slow dances ("You're very light on your feet.") or when the robot falls in love with an ordinary houselamp.
Lurking in the shadows of the Dark Ages is the howling form of Grendel. He is the monster of midnight, the bone-gnasher, the ardent hunter of warriors who strews their bones and howls his fury to the world as he wreaks havoc on the safety of civilization. No hall fire burning brightly, no line of armed men can keep him back when he desires destruction. But as John Gardner tells of Grendel, this was not always so. For the bane of the Hrothgar’s hall has a soul much tormented by his desire for good and fellowship with the humans even as his demonic appearance frightens them into violent action. To them, he is a thing, and so he becomes what they believe him to be--an adversary whose fame has spanned the centuries.