When a sick lion shows up at Jerome's monastery, the saint takes care of it and it stays to become his companion after it recovers. The monks, resentful at giving up their scarce meat to an animal, insist that the lion be put to work. The lion tries to guard the donkey, but it is not leonine habit to stay awake all day and eventually the valuable pack animal is stolen by traveling merchants. Miserable and ashamed, the lion must bear the donkey's burdens in her place, until the merchants pass that way the following year and he can rescue the other animal.
This is a different version of the legend of St Jerome and the lion than the one I am familiar with, which features a more solitary encounter between beast and saint while Jerome is a hermit in the desert. A note at the beginning attributes this version to "a Latin manuscript of the Middle Ages" via Migne's Patrologia (Vol 22).
Moser's illustrations were of a high quality, and I thought he did a particularly good job capturing the darkness of the pre-modern night, in a way that is rarely to be seen in picture books. He mentions both Durer and Carpaccio as sources for the imagery.