It's an unpleasant book, deliberately so. It's the POV of an annoying little shit with a lot of mental problems. He's an irritating character, so it's not fun to spend time with him.

This book is similar to Lolita in that both books put you inside the head of an unpleasant person. Of course, in the case of Humbert Humbert, he's evil and criminal, as well as unpleasant.