Picture Book, Children's Literature
"Children's Literature" is usually defined simply by the target audience: it's for kids younger than ten years old or so. That means there's a wide variety of things that fall into it: water-proof baby bath books, illustrated accounts of the civil rights movement, and fictional picture books. The latter is definitely the most prominent, and it's what Alexander falls into perfectly. In fact, Alexander remains one of the quintessential picture books for parents, educators, critics, and librarians the world over.
Picture books usually have one central idea (plot twists and secondary narratives are a little too much for the kiddos to keep track of) and they tend to be around 30 pages long. Their distinctive feature is the relationship between the words and the pictures. That's right, "relationship."
The pictures don't always—in fact, they rarely ever—just show what the words say. Usually, they add information, fill gaps in the text part of the story, and add tone, ambiance, and feeling. For example, when Alexander innocently narrates that in his dad's office he was "as careful as could be except for my elbow" (p. 20), the picture shows papers flying all over the place, a broken flower pot, and two exasperated parents. It's the picture that tells us what the words are really about.
Because the pictures are such a major part of the narrative, kids can do a lot of thinking about the book on their own. They can take time looking at the pictures and noticing details that aren't in the text—the part of the story that they either can't read on their own or might be hard to read as they're learning.