“To study the phenomenon of disease without books is to sail an uncharted sea, while to study books without patients is not to go to sea at all.”

Source: The Memorial and Instructions, p. 302

Source: Crome Yellow (1921), Ch. XXVIII

Source: Evangelical Theology: An Introduction

First Week, Seventh Day.
La Semaine; ou, Création du monde (1578)

Letter to Oskar Pollak http://www.languagehat.com/archives/001062.php (27 January 1904)
Variant translations:
If the book we are reading does not wake us, as with a fist hammering on our skulls, then why do we read it? Good God, we also would be happy if we had no books and such books that make us happy we could, if need be, write ourselves. What we must have are those books that come on us like ill fortune, like the death of one we love better than ourselves, like suicide. A book must be an ice axe to break the sea frozen inside us.
What we need are books that hit us like a most painful misfortune, like the death of someone we loved more than we love ourselves, that make us feel as though we had been banished to the woods, far from any human presence, like a suicide. A book must be the ax for the frozen sea within us.
A book should be an ice-axe to break the frozen sea within us.
A book must be an ice-axe to break the seas frozen inside our soul.
A book should serve as the ax for the frozen sea within us.
Variant: A book must be the axe for the frozen sea inside us.
Context: I think we ought to read only the kind of books that wound and stab us. If the book we are reading doesn't wake us up with a blow on the head, what are we reading it for?... we need the books that affect us like a disaster, that grieve us deeply, like the death of someone we loved more than ourselves, like being banished into forests far from everyone, like a suicide. A book must be the axe for the frozen sea inside us.

Preface, 1928
The Story of San Michele (1929)

Genius; reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 88.