#bookaday is one. Unlike John, I wasn't very good at French, but when we studied Les Mains Sales by Jean-Paul Sartre I (a) loved it and (b) got straight As for my GCSE essays about it.
As a result of this surprising development I took French at A-Level, and after that at degree level, which is bizarre given that I was never any good at understanding or speaking the language. The oral exam for my degree went so badly that bellows of laughter broke out as I left the room! I used English words at one point!
The mistake I'd made after the great scores for those GCSE essays about Les Mains Sales was thinking I was good at French, whereas in fact it was the literature bit I liked. And because of that mistake I took a literature degree in a foreign language, with all the inconvenience one might expect from such a course of action.
But credit to Les Mains Sales: it got me studying literature, which had never been part of my plan, and the literature degree helped get me into publishing, and I'm doing okay in my quiet little way. For one thing, I've been able to work from home for the first decade of our having children, and I feel incredibly fortunate for that.
So thanks Les Mains Sales. I wish you hadn't derailed my education, but thanks for getting me into reading books in French, and thanks for getting me where I am today!
Honourable mentions: Of Mice and Men by John Steinbeck, The Pinballs by Betsy Byars, and A Midsummer Night's Dream by William Shakespeare.