These are books that I enjoyed much more than I anticipated. Needless to say I recommend them.
Below are some of the reasons (with examples):
I didn't know what to expect (Age of Innocence, The Grammarians, Washington Square)
I had been forewarned that the book was heavy going, when it didn't turn out to be so (Moby-Dick, The Tin Drum, The Scarlet Letter)
A previous book by the same author was disappointing (Breakfast of Champions, Cat's Cradle, Galapagos after Slaughterhouse Five)
Not run-of -the-mill for that genre (Shel Silverstein's works, Harold and the Purple Crayon, The Phantom Tollbooth)
Far more humorous than anticipated, probably the most common reason (Lolita, Northanger Abbey, Candide)
Moving and insightful (Silas Marner, Old Man Goriot, Cloudstreet)
Gave insight into other parts of the world and/or historical periods (1914, Things Fall Apart, Every Day is For the Thief, A Fine Balance)
A savage parody of academic life and intellectuals (Stoner, Pnin, Clouds).