OP is probably referring to the types of books generally supplied to small children who are first learning how to read. They tend to be extremely short (no more than a few dozen words in the whole book); heavily illustrated; and use simple words and sentence structure.

By extension, I gather they’re looking for examples of well-engineered programs small and simple enough for a true beginner to thoroughly grok within a short period. Presumably something in a few hundred lines of code that avoids unnecessary indirection or cleverness.