I begin a unit on alphabet books by reading Chris VanAalsburg's book "The Z was Zapped". Then I have each of my students pick a letter, brainstorm new ideas, and create their own page. This is bound together for a class book.
Then I spend a few days looking at all different kinds of alphabet books.
After this, we brainstorm possible topics. Each student picks a topic and begin writing. Besides coming up with a noun for each letter, they also must write a sentence. For example: R is for robins. Robins have red breasts.