One of my favorite children's book collections is the "Nutshell Library" originally published in the 1960s by Harper & Row. Each set includes 4 nutshell-sized (3.9 x 1.8 inch) picture books that were illustrated by one of 3 masters of children's picture book art: Sendak, Knight, and Kraus.
Maurice Sendak's "Nutshell Library" was published in 1962:
Hilary Knight's "Christmas Nutshell Library" was published in 1963:
via Page Books
Robert Kraus's "The Bunny's Nutshell Library" came out in 1965:
Sadly, only Sendak's "library" is still in print, but if you see any of these sets or even any of the books individually - buy them up! (especially if you find Knight's collection WITH book jackets on the cheap. It is one of the things that, like These are all beautiful books and the mini-ness of the format just makes them that much more lovely to behold as well as hold.
from Sendak's "Pierre" (via Little Lamb Books)
from Knight's "A Christmas Stocking Story"
from Kraus's "The Silver Dandelion" (via Vintage Kids' Books My Kid Loves)