The Turn of the Screw
Henry James's governess, and what she may or may not have seen.
86 cryptograms drawn from it, every one by Henry James.
How hard they are
28 easy, 35 medium and 23 hard, graded by the model described in our guide to solving cryptograms.
They average 70.7 letters over 19.1 distinct ones, with 4.3 letters appearing exactly once.