The note to Alexander Pope's Imitations of Horace quotes from Joseph Spence's yet-unpublished Anecdotes. Pope appears to be recalling Abraham Cowley's account of his early reading of the Faerie Queene as described in Of Myself.
George Steevens to Thomas Percy: "Concerning Dr. Warton's edition of Pope, you may read the first article in the Monthly Review for August last. I wish our venerable friend had not undertaken this work at so late a period of his life. But, though it will add little to his reputation, for his trouble he received no less a sum than five hundred pounds" 9 September 1797; in Nichols, Illustrations (1817-58) 7:30.
Samuel Egerton Brydges: "In 1797 he published the works of Pope in 9 vols. 8vo. The notes to this edition, which necessarily include the greatest part of his celebrated Essay, are highly entertaining and instructive. But Dr. Warton was severely, and, it may be added, illiberally, attacked for inserting one or two somewhat indecent pieces in this edition, which had hitherto been excluded from his collected works. The most harsh of these attacks came from the author of the Pursuits of Literature [Thomas James Mathias]: something, no doubt, must be deducted from the violence of one, whose professed object was satire; but the grey hairs and past services of Warton ought to have protected him from excessive rudeness; and these over-nice critics might, with a proper regard to consistency, have demanded the exclusion of several other works of Pope. It must not be concealed, however, that Beattie agreed in some degree with these censurers" Censura Literaria 3 (1807) 192.
Mary Russell Mitford: "I have, since writing to you, been reading all the readable books of these great poets — Dryden in Walter Scott's edition of eighteen volumes, and Pope in ten huge volumes edited by your friend Mr. Bowles, with notes by Warburton, by Warton, by Walpole, by Ruffhead, by Steevens, by Malone — in short, by every body; at the rate of two lines of Pope to twenty of prosing. I am not a little delighted with both authors, and a great deal disgusted with their editors" 24 December 1815; in L'Estrange, Life of Mary Russell Mitford (1870) 1:247.
P. W. Clayden: "Warton had just then  finished his edition of Pope, on which it is said he had been engaged for sixteen years. He was in his old age. He had resigned the head mastership of Winchester School in 1793, and was living at Wickham, of which parish he was rector, where he died in February 1800" The Early Life of Samuel Rogers (1887) 355n.
How much this author was his favourite from his early to his later years, will appear from what he said to Mr. Spence, from whose Anecdotes I transcribe literally this passage "There is something in Spencer that pleases one as strongly in one's old age as it did in one's youth. I read the Fairy Queen when I was about twelve with a vast deal of delight; and I think it gave me as much when I read it over about a year or two ago."