In brief: Eric wants a dog; instead, he gets Karlson, a grumpy man who flies around with his propellor making mischief.
What makes it a great readaloud? Karlson is utterly unlikable—in the great trickster tradition, he wreaks havoc (and usually leaves a mess) wherever he goes. The gently interconnected stories are weird and often hilarious, and 7-year-old Eric (also known as Smidge or Little Brother, depending on your translation) is charming protagonist.
But be aware: Karlson is a grouchy trouble-maker and may not appeal to every reader.