Friedrich Nietzsche on Dangerous Books

An extract from Assorted Opinions and Maxims
Georg Friedrich Kersting, 'Man Reading at Lamplight' (1814)
From Friedrich Nietzsche's Assorted Opinions and Maxims, translated by R. J. Hollingdale:
Dangerous books. - Somebody remarked: 'I can tell by my own reaction to it that this book is harmful.' But let him only wait and perhaps one day he will admit to himself that this same book has done him a great service by bringing out the hidden sickness of his heart and making it visible. - Altered opinions do not alter a man's character (or do so very little); but they do illuminate individual aspects of the constellation of his personality which with a different constellation of opinions had hitherto remained dark and unrecognisable.
Also at A Piece of Monologue: