From Horace Walpole’s January 28, 1754 letter to Horace Mann, describing the background of a heraldry-related discovery. This passage is the first instance of the word serendipity.

This discovery I made by a talisman, which Mr. Chute calls the sortes Walpolianae, by which I find everything I want, a pointe nommee [at the very moment], wherever I dip for it. This discovery, indeed, is almost of that kind which I call Serendipity, a very expressive word, which, as I have nothing better to tell you, I shall endeavour to explain to you: you will understand it better by the derivation than by the definition. I once read a silly fairy tale, called the three Princes of Serendip: as their Highnesses travelled, they were always making discoveries, by accidents and sagacity, of things which they were not in quest of: for instance, one of them discovered that a mule blind of the right eye had travelled the same road lately, because the grass was eaten only on the left side, where it was worse than on the right—now do you understand Serendipity? One of the most remarkable instances of this accidental sagacity (for you must observe that no discovery of a thing you are looking for comes under this description) was of my Lord Shaftsbury, who happening to dine at Lord Chancellor Clarendon’s, found out the marriage of the Duke of York and Mrs. Hyde, by the respect with which her mother treated her at table.

I’m reading Robert Merton (of On the Shoulders of Giants fame) and Elinor Barber’s The Travels and Adventures of Serendipity: A Study in Sociological Semantics and the Sociology of Science and the most (personally) interesting aspect of the word’s history is its early adoption in bibliophilic circles for the feel of small but vital delight when you unexpectedly meet a work you’d earlier sought but hadn’t found or, for me, one you hadn’t even realized you’d been wanting.