My children are 6 1/2 and 4 1/2, so we don't have many past the 4th grade level yet (aside from my large personal collection of books). This post contains books good for up to a first grade level (at least in our house) to read on their own.
I've ordered the books from easiest reading to more advanced.
My youngest loves reading Monster Town by Ryan Heska. This book follow a young vampire boy on a tour through his home, Monster Town. The colorful illustrations depict the inhabitants, like the Ghost Writers at the news paper, Dr. Mummy the vetrinarian, and the Giant Squid, proprietor of the local cafe.
"In the cold gray tomb, there was a gravestone and a black lagoon, and a picture of ...." Goodnight Goon by Michael Rex is a playful parody on the chidren's classic, Goodnight Moon, in which a young werewolf is getting ready for bed surrounded by classic horror icons. The illustrations are make what may initially seem scary to children something fun.
Another playfully dark Michael Rex book is his parody of Runaway Bunny, called Runaway Mummy. It shows that even scary monsters like mummies can be a loving and good family.
The Spider and the Fly illustrated by Tony DiTerlizzi (author of the Spidewick Chronicles). The book is made to look like silent black and white film set in a Victorian doll house. The tale by Mary Howitt is timeless and teaches a good lesson, but the illustrations are what really make it engaging. My children have spent hours just looking at the pictures in this book.
Step right up and enter the Creature Carnival by Marilyn Singer and Gris Grimly. This trip through the traveling side show introduces children to fantastic mythological and fabled creatures, such as the Lochness Monster, Sirens, the Minotaur and Anansi. The story and pictures have a playful yet macabre air about them. There are more Gris Grimly books in my children's future.
Hipira the Little Vampire by Katsuhiro Otomo is a lot of fun. This book is presented in comic book style and contains several stories in which Hipira, a little vampire boy and his best friend, a sprite named Soul get into all sorts of harmless mischief. While there is no real message in this book, it is pure fun, and a great way to introduce your little ones to vampires.
Cinderella Skeleton by Robert D. San Souci takes the classic tale of Cinderella casts it with the undead and transports it to a graveyard setting. The story is presented in a collection of sonnets and shows the skeletons can be as wonderful as you and me.
Frankenstein Makes a Sandwich and Frankenstein Takes the Cake, both by Adam Rex. These whimsical books are filled with short stories, poems and songs all with classic monster themes and accompanied by original artwork. When we first got this, we had to read it to Lily every night for months. Even now, three years later, she still frequently reads this book before bed.