I will not even blame Mr. Horowitz for the Latin error on page 10, since it seems to be a typographical error. "Quid te exempts iuvat spinis de pluribus una?" should really be "Quid te exempta juvat spinis e pluribus una?" ("Better one thorn plucked than all remain"). I'm sure this bothered all of you as much as it bothered me.
Thank you to Michael Gilleland's site for answering this question.
Nota Bene: T.H. White's Latin in The Once and Future King (1939) is flawless as far as I can tell. Sic transit gloria mundi.