Jean Ingelow, a poet of the 19th century, gets a few pages here, tucked between chapters that run only slightly longer. It sounds like she was among the many Victorians to turn their hand to fantasy tales for children, her great work being Mopsa the Fairy.
From how Hollander writes of the story, it sounds like a standard Victorian fairy tale: children find a fantastic being in real life, go to Fairy Land, sort something out there, and then return, changed. However, that Ingelow included a doppelganger for her hero, Jack, reminds me of Dark Link from the Legend of Zelda games.
And really, why not? When it comes to quest stories, what better enemy can a hero have than his or her shadow?
After all, to go on a quest, a person needs to be able to endure long miles, make do without the comforts of home, face unfamiliar situations, and do things he or she may not otherwise do. You're fighting your shadow from the first step of any journey, so why not turn that into a villain or foil?
Hollander doesn't get into shadows and doubles, though. Instead, he brings up Ingelow to savour her poem "Failure." A sonnet about failure being more interesting than success that turns on the story of Orpheus and Eurydice. It's an entrancing piece that is, itself, almost a portal to another world, another time. If you're curious, or adventurous, give it a read here.