Suite 101 has a great section on writing children’s books and categories of books. This list of book categories is based on their work.

Picture Books — A picture book is a book in which the illustrations play a significant role in telling the story. There are several types of books under this category:
1. Baby books —These books are for infants and young toddlers, and are usually lullabies, nursery rhymes, fingerplays or wordless books. The length and format can vary with the content.

2. Toddler books — These are very simple stories for ages one to three, usually under 300 words. They are familiar to a child’s everyday life, or they are concept books which teach colors, numbers, shapes etc. Books are short with a 12-page average.

3. Picture books — Traditionally, picture books are 32-page books for ages four to eight. Manuscripts are up to 1500 words, with 1000 being the average length. Plots are simple with one main character who embodies the child’s emotions, concerns and viewpoint.

4. Early picture books — This is a term for picture books geared toward the lower end of the four-to-eight range. These stories are simple and contain under 1000 words. Many early picture books have been reprinted in the board book format.

Easy Readers — These books are also called “easy-to-read” and are for children just starting to read on their own (ages 6-8).

Transition Books — These books are sometimes known as “early chapter books.” They are written for ages six to nine and bridge the gap between easy readers and chapter books.

Chapter Books — Written for ages seven to ten, these books are 45-60 manuscript pages long, and are broken into three- to four-page chapters. The stories have more substance and contain a lot of action.

Middle Grade Books — This category is referred to as “The Golden Age” of reading and is aimed at children eight to 12 years. Manuscripts are longer (100-150 pages) and the stories are more complex.

Young Adult — Written for ages 12 and up, these manuscripts are 130 to about 200 pages long. Plots can be complex with several major characters, though one character will emerge as the focus of the book.