Well your level of reading has to start somewhere, right?? In Pit's case, he knows about as much as a preschooler... Aziraphale would likely always view Pit as a small child, at least until he reaches one hundred in human years, because fifty is so incredibly young for an angel.

Aziraphale sits down beside him and opens up the book. "There are 26 letters in the English alphabet. Each one represents a different sound, and letters can be combined to make different sounds, and even full words. The first thing to do is to learn all of these letters, every sound they make, and how to write them."

Obviously the first letter in the picture book is the letter A, and there's a stereotypical apple above it... although this one may or may not have a snake curled around its base. What? Aziraphale could take some liberties for the sake of his own amusement...