The Far-Distant Oxus

I read The Whispering Mountain by Joan Aiken and was struck by similarities to Pratchett, for instance the part where the main character heroically defeats monsters in a wood by using knowledge gleaned from an old encyclopedia that he carries everywhere, and how he ſpeakſ like thiſ when reading aloud from it, and the part about underground camels in Wales. It references The Far-Distant Oxus at one point, which I want to read (a pony adventure story written in 1937 by teenagers). (I know the long s wasn't really used at the ends of words, that was just a hurried example.)