Which book? As if there were only one?
I read these books annually, and so have read them each at least 10 times:
The Riddle of the Sands, by Erskine Childers
The Falcon on the Baltic, by E.F. Knight
The Complete Memoirs of George Sherston, by Siegfried Sassoon
An incomplete list of books I have read at least 4 times would include:
The Annotated Alice, by Lewis Carroll and Martin Gardner
The Chain of Chance, by Stanislaw Lem
The Cyberiad, by Stanislaw Lem
His Master's Voice, by Stanislaw Lem
The Investigation, by Stanislaw Lem
Zeppelin Adventures, by Rolf Marben
The Old Left, by Daniel Menaker
The Road Back, by Erich Maria Remarque
My Airships, by Alberto Santos-Dumont
The War Poems of Siegfried Sassoon, by Siegfried Sassoon
The Fellowship of the Ring, The Two Towers, The Return of the King, by J.R.R. Tolkien
The Hobbit, by J.R.R. Tolkien
Mike at Wrykyn, by P.G. Wodehouse
Mike and Psmith, by P.G. Wodehouse
Psmith in the City, by P.G. Wodehouse
Psmith Journalist, by P.G. Wodehouse
Leave it to Psmith, by P.G. Wodehouse
I read a lot of science fiction books multiple times as a teenager, but I have no idea now which I would have read at least 4 times.
Pretty much, if I "don't know what to read next", instead of posting a question here :) I just look at my bookshelf for something I've enjoyed in the past.