Theodor Fontane's novel Effi Briest is often taught at German schools. The Wikipedia article about the novel has a section on the novel's legacy which says,
[Thomas] Mann said that if one had to reduce one's library to six novels, Effi Briest would have to be one of them.
The source for that statement is identified as "Mann, Thomas (1968). Das essayistische Werk". However, that is actually a set of eight volumes. (The Spanish Wikipedia article about the novel also contains this claim, but no source is given.) Which specific essay or publication included in that work contains the claim mentioned above?
(I have worded the question as "... in his library", even though the quote does not seem to refer to Mann's own book collection, because I assume that the books he has in mind were actually in his library. However, I consider this detail less important than the actual source of the statement.)