Sorry for the awkward title.
These days we have all sorts of simple books to help kids learn to read. I am thinking of the type with a big picture of an apple with the words "A is for.." underneath.
Are there any examples of similar types of books from, say, the medieval period when reading was much more limited?
Yes, absolutely!
I know this sounds silly, but, I give an overview of the prehistory of children's literature in the original answer and a couple of follow-ups to, "Did kids in the 1800s American West know how to make exotic animal noises, like a lion's roar?"
There are a few missing pieces, like the use of the Psalter (Book of Psalms) as the basic textbook for Latin literacy in medieval monasteries, but in general--hornbooks is the major answer for Europe.
I hope that answer helps a little!