This is a minor novella, written as a post-Abhorsen story. It can be found in the story Across the Wall: A Tale of the Abhorsen and Other Stories . It features Nicholas Sayre's adventures Across The Wall, as he tries to return to the Old Kingdom.
Copied from Wikipedia Across the Wall: A Tale of the Abhorsen and Other Stories is a collection of short stories by Garth Nix, which return to the setting of his popular Old Kingdom series. A hardback edition was released in the UK on November 6, 2006. There are two special editions of this book in the UK, one with "a unique cover" from WH Smith's, the other with a manuscript for a possible prologue and several other notes on the story of Lirael, available from Waterstone's. All UK editions will include an FAQ section with the author, in which he provides "vital answers". All thirteen main stories had been published prior to the release of Across the Wall, usually in magazines or anthologies.