best book
A an example quote, highlighted by Libb Thims (2006), of Goethe’s circa 1820s “best book” comment, as described by Helmut Muller-Siever (1997), as found in Astrida Tantillo’s 2001 Goethe’s Elective Affinities and the Critics. [6]
In publications, best book refers to the self-defined “best” publication by German polymath Johann Goethe, among his prodigious literary output of 142 plus collected works set—which, in terms of world literature library representation, ranks second only to that of Shakespeare—namely his 1809 physical chemistry based novella Elective Affinities.

Overview
Of Goethe's 142 plus collected works publications, his self-defined “best book” was his 1809 Elective Affinities. [1] As summarized by American musicologist Kristina Muxfeldt, it was a “work he counted among his greatest achievements”. [2] Noted Elective Affinities critique Walter Benjamin (1892-1940) notes “the speechless irony” of a scene reported by German dramatist, novelist and theatre-director Heinrich Laube: [3]

“A lady addressed Goethe on the subject of Elective Affinities: ‘I do not approve of this book at all, Herr von Goethe; it is truly immoral and I do not recommend it to any women.’—Thereupon Goethe kept a serious silence for awhile, and finally, with great civility, replied: ‘I am sorry, for it is my best book.’”

A fuller version of the incident, as recounted in the 1949 Goethe: the Story of a Man: Being the Life of Johann Wolfgang Goethe as Told in his Own Words and the Words of his Contemporaries, a collection of letters and correspondences, edited together by Ludwig Lewisohn, is as follows: [4]

“A women friend of mine said to Goethe at that time: ‘I cannot approve of Elective Affinities, Herr von Goethe; it really is an immoral book!’ According to her report Goethe was silent for a while and had then said with great earnestness: ‘I’m sorry you feel that way. It is my best book, and don’t think that this is the mere whim of an aging man. I grant you that one loves most deeply the child of one’s last marriage, the product of one’s late power of generation. But you wrong me and the book. The principle illustrated in the book is true and not immoral. But you must regard it from a broader point of view and understand that the conventional moral norms can turn into sheer immorality when applied to situations of this character.”
Heinrich Laube
German dramatist Heinrich Laube, source for the Goethe
circa 1809-10 "best book" incident.

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Date: 1809 or 1810?
The previous version by Lewisohn is dated as "late December", 1809.

American Goethean scholar Alfred Steer, in his 1990 Goethe’s Elective Affinities: the Robe of Nessus, however, reports that the incident took place in 1810, as follows: [7]

“In 1810, as Heinrich Laube reports (1), a lady is supposed to have complained to Goethe about the immorality of the novel; the author is said to have replied: "I am sorry, but it is after all my best book.”

The Steer (1) reference, here, then, disagrees with the Lewisohn (1949) reference.

The original Laube source thus needs to be tracked down.

Beyond this, if the incident was passed along through word of mouth, as it seems to have been, and first documented by Laube, this would imply that he would have had to have been at least the age of adult hood, circa age 20 (1826), to have lady friends bold enough to reproach Goethe who would later tell him such an incident.

Other
To corroborate the above view that Goethe held the deep opinion that Elective Affinities was his greatest work, in 1827 he commented to his literary associate German author Johann Eckermann: [5]

“The only production of greater extent, in which I am conscious of having labored to set forth a pervading idea, is probably my Elective Affinities.

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See also
‚óŹ Goethe timeline

References
1. Classe, O. (2000). Encyclopedia of Literary Translations into English (quote: “my best book”, pg. 543). Taylor & Francis.
2. Muxfeldt, Kristina. (2011). Vanishing Sensibilities: Schubert, Beethoven, Schumann (pgs. 153-54). Oxford University Press.
3. (a) Benjamin, Walter. (1999). Selected Writings, Vol. 2, 1927-1934 (pg. 328), ed. Howard Eiland, Gary, Smith, and Michale W. Jennings. Harvard University Press.
(b) Walter Benjamin – Wikipedia.
4. Lewisohn, Ludwig. (1949). Goethe: the Story of a Man: Being the Life of Johann Wolfgang Goethe as Told in his Own Words and the Words of his Contemporaries, Volume 2 (pg. 174). Farrar Straus and Co.
5. (a) Tantillo, Astrida, O. (2001). Goethe’s Elective Affinities and the Critics ("Conversations with Eckermann" (06 May 1827) pgs. 154-57). New York: Camden House.
(b) Conversations with Goethe (Gespräche mit Goethe) – Wikipedia.
6. (a) Muller-Sievers, Helmut. (1997). Self-Generation: Biology, Philosophy, and Literature around 1800. Stanford University Press.
(b) Tantillo, Astrida, O. (2001). Goethe’s Elective Affinities and the Critics (pg. 198). Camden House.

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