Charles Bewitched. Leland Sisters, #4

They'll never believe him back at school when he tells them how he spent his summer holidays.Sixteen-year-old Charles Leland is not looking forward to his summer holidays from Eton-not when he has to spend them cramming history to make up for a less-than-stellar rank last term. Even the thought of staying with his sister Persy and her husband Lochinvar while his parents are in Ireland can't cheer him up.
But studying is forgotten when Persy disappears from home the day he arrives. All signs indicate that she's been kidnapped, but Charles is certain that her disappearance has a much more otherworldly explanation. Now Charles must brave the perils and sheer strangeness of the fairy lands to prevent his sister from being forcibly married to a powerful fairy lord, with only the help of the fairy lord's own younger (and alarmingly pretty) sister and a copy of History and Policy of the Norman and Angevin Kings that he must read before September.
Help will arrive from an unexpected source-but will he also be able to rescue himself?