"It can be accounted for only in this way; that by reading and meditation, and a very close inspection of life, he had accumulated a great fund of miscellaneous knowledge, which, by a peculiar promptitude of mind, was ever ready at his call, and which he had constantly accustomed himself to clothe in the most apt and energetic expression."
— Boswell, James (1740-1795)
(I, p. 109; p. 114 in Penguin)
See The Life of Samuel Johnson, LL.D. Comprehending an Account of His Studies and Numerous Works, in Chronological Order; a Series of His Epistolary Correspondence and Conversations With Many Eminent Persons; and Various Original Pieces of His Composition, Never Before Published. The Whole Exhibiting a View of Literature and Literary Men in Great-Britain, for Near Half a Century, During Which He Flourished. In Two Volumes. By James Boswell, Esq. 2 vols. (London: Printed by Henry Baldwin, for Charles Dilly, in the Poultry, 1791). <Link to ESTC><Vol. I in ECCO-TCP><Vol. II>
My main reading text is James Boswell, The Life of Johnson, ed. Claude Rawson, (New York: Knopf, 1992). Also reading in David Womersley's Penguin edition, 2008.
First edition in Google Books, <Vol. I><Vol. II>. See also Jack Lynch's online e-text, prepared from the 1904 Oxford edition <Link>.