Definition of "improving"

[…] that Diviner part of Man, the Soule, which alone is capable of wearing the Glorious Image of its Author, being endowed with two chief Faculties, the Understanding and the Will; the former is blest and perfectionated by Knowledg, and the latter’s Loveliest and most improving property is Goodnesse.

1663, Robert Boyle, Some Considerations Touching the Usefulnesse of Experimental Naturall Philosophy, Oxford: Ric[hard] Davis, Essay 1, p. 2

[…] nothing is so improving to the Temper as the Study of the Beauties, either of Poetry, Eloquence, Musick, or Painting.

1741, David Hume, “On the Delicacy of Taste and Passion”, in Essays, Moral and Political‎, Edinburgh: A. Kincaid, page 6

She was so overcome by the splendor of his achievement that she took him into the closet and selected a choice apple and delivered it to him, along with an improving lecture upon the added value and flavor a treat took to itself when it came without sin through virtuous effort.

1876, Mark Twain, chapter 3, in The Adventures of Tom Sawyer‎

To boast mendaciously about one’s own gang and to slander and defame other gangs are acts everywhere officially regarded as creditable and even pious. It is as though our rulers, instead of merely tolerating prostitution, were to proclaim the brothel to be a place as sacred as the cathedral and as improving as the public library.

1934, Aldous Huxley, “Guatemala City”, in Beyond the Mexique Bay‎, London: Chatto & Windus, pages 95–96

“Nothing to do but keep the chin up and the upper lip as stiff as can be managed. I think I’ll go to bed with an improving book. […] ”

1954, P. G. Wodehouse, chapter 4, in Bertie Wooster Sees It Through‎, New York: Scribner, published 2000, page 45