Publishers of children's literature generally break down their books by age group. By their second birthday, children are just learning what a book is, while middle-school children are reading so-called chapter books. For the writer, this broad spectrum of book types and age groups presents many opportunities, both in fiction and nonfiction.

Picture Books

The picture book category has a number of subcategories, again determined by the child's age group. “Baby books” are picture books for infants and young toddlers, and most are designed to assist some aspect of the child's development, such as the motor skills involved in holding a book and the cognitive skills to turn pages in order. Baby books may be wordless or may include very short stories like nursery rhymes.

So-called “toddler books” are for children ages one to three. These books also are short on text — usually 300 words maximum — and often involve learning concepts like simple counting, identifying familiar animals, and so on. The average toddler book is twelve pages long and may be produced in a variety of formats; the most common is the “board” book, in which the pages are constructed of sturdy cardboard.

The age range for other types of picture books varies by publisher, but usually they are meant for three- to six-year-olds. Stories for these books are lengthier, typically between 1,000 and 1,500 words. The story should be told through one main character, and that character should be one that very young children can identify with. Illustrations still carry much of the story in these books, and the plot, though simple, requires lots of action.

Easy-Read Books

Easy-read books are aimed at children who are just beginning to read by themselves, usually between ages six and eight. These books still have lots of illustrations (often one on every page), but they are formatted more like books for older children. The reason for this is simple: six- to eight-year-olds are beginning to feel like “big kids,” and they aren't interested in books that look like they're meant for babies.

Easy-read books can vary greatly in length, often depending on the publisher. Some of them are as short as 200 words; others are as long as 2,000 words. These books aren't as simplistic as the old “See Spot Run” primers many of us grew up with, but they should be written in short sentences that address one idea at a time. There may be two to five sentences per page. As with picture books, easy-read books require a single main character and lots of action.

Chapter Books

Designed for the six- to nine-year-old and sometimes called “transition” books, early chapter books form a bridge for young readers between illustration-heavy picture books or easy-read books and more text-heavy chapter books. The style for these books is similar to the easy-read style: one main character in an action-packed plot, told in short, simple sentences. These books still have illustrations, but they are usually black-and-white drawings, and they are more scattered. Chapters are typically two to three pages long, and an entire book will run around thirty pages in manuscript format.

Chapter books for readers ages seven to ten are a little longer — between forty-five and sixty manuscript pages — and have slightly longer chapters, usually three to five pages. The stories for this age group are a little more complex, and the sentence style also can be more complex, but paragraphs usually are no more than four sentences long. “Cliffhanger” chapter endings, or endings in the middle of a scene, are common, designed to keep the reader turning pages.

Middle-Grade Books

Middle-grade readers are eight to twelve years old, and they tend to read lots of different types of books. Favorite fiction genres fill the genre spectrum, from mystery to science fiction, contemporary, and historical. Also popular with middle-grade readers are series books, which feature the same characters, either in different stories or in a continuing saga like the Lemony Snicket books.

Middle-grade books make a great leap from early chapter books, both in length and in complexity. Manuscripts burgeon to between 100 and 150 pages. Sentence structure is more complex. Subplots and secondary characters weave in and out of the main story, and themes become more sophisticated.

Young Adult Books

The “young adult” or YA category is one of the most fluid in children's books. Some publishers consider their YA lists suitable for any reader between the ages of twelve and nineteen. Others have broken this category down into two sets: young teenagers (ages ten to fourteen) and young adults (ages twelve to sixteen).

This category is the last demarcation between children's literature and adult literature. Themes, plots, and subject matter all are more complex in these books than in earlier categories, though some child-appropriate sensibilities still apply. YA publishers often have specific guidelines, especially for fiction. For example, a young teen romance may culminate in a kiss, but rarely will a publisher accept more explicit physical scenes for this age group.

Hi/Lo Books

Hi/lo books are those aimed at children whose interest in a topic is high, but whose reading skills are below average for their grade level. Generally, these stories are simpler than the typical YA book, but they are fast-paced with lots of dialogue. Fiction books in this category usually have contemporary settings and realistic dilemmas. Nonfiction hi/lo books deal with hot topics that sixth- through twelfth-graders are interested in.