Lolly Willowes; or The Loving Huntsman, by Sylvia Townsend Warner
Starts of like a very well observed, fairly genteel, faintly feminist, Edwardian novel about a youngish woman who has to go and live with her brother after the death of their father. Fairly light, perfectly enjoyable, and then...there is just one line, which you read, pause, go 'wtf?' and reread, and it realy said what you thought it did. And then it's back to very well observed, comparatively genteel, slightly less faintly feminist, Edwardian novel. Then it get's a bit weirder, and then a bit weirder still.
If you read anything about it - including the literary Introduction, or any one sentence review, or, probably, the back cover, you discover smething which is much better left undiscovered, imo. The 'wtf?' is a real doozy of a 'wtf?' in that instance.
Bloody brilliant book, a must for anyone who likes words written down on a page.