In late July 1982, Philip Larkin wrote one of his last poems, Long Lion Days. From most drying nibs, 10 seasonal lines in rhyming couplets would plod to obscurity. This poet signs off with an unending summer bounding into elemental realms of fire: “Whatever conceived Now fully leaved, Abounding, ablaze - O long lion days!” How do such poems come about? What is their inspiration? In our hypertextual age of cut and paste, surf and spin, we’re not supposed to ask unfashionable questions about the genesis of literary works. Yet we still want to track back to authorial origin: to drink long from Keats’s heady beaker full of the warm south, to feel the heat of the furnace in which Blake forged his immortal tiger.

Larkin knew one way to get close to creative detonation.

Its earliest aftershocks could be read in what he described as the “magical value” of an author’s manuscript. Here, as he put it in an essay for fellow librarians, we see “the words as he wrote them, emerging for the first time in this particular miraculous combination... It is a potent element in all collecting and I doubt if any librarian can be a successful manuscript collector unless he responds to it to some extent”.

The Bodleian Library recently fell under the spell when it acquired the poet’s series of letters and poems to Monica Jones, an unknown contemporary at Oxford who would laterbecome his most intimate friend and confidante. I say “letters” because it is an instinctive reaction of manuscript librarians like me to ignore what gets shoved to the bottom of the archival pile, along with the gas bills, shopping lists and old TV licences - that is, the greetings cards and postcards.

Yet it was while rummaging through one of many boxes of these flimflams - a temptress in a negligée with Monica specs penned in; fluffy fauna alluding to icky pet names; a hedgehog, possibly in memory of the one he mangled in a mower - that Larkin caught me in an abracadabra moment.

On the back of a yellowing birthday card, he had written a poem. This brought two nice surprises. First, it was unknown vintage Larkin. (It is published here for the first time.) Second, it seemed to open up a new way of understanding the poetry. We are all familiar with Larkin the gritty scribe, able to capture everyday scenes through nothing more opaque than the windows of his bachelor digs, library office or swerving train carriage, before launching into airy realms of beauty.

Yet here, in phrases redolent of his greatest work, the poet responds not to what he has seen in the real world or the mind’s eye, but to a picture. That picture, reproduced on a card (above, second from right), is a delicate 1831 engraving of Goodwood races, where “long thin steeds” run before a “tiny” crowd whose “excitement” is confined to a “delicate stand” situated near “natural downs”. Larkin probably never offered his poem to press because the text so intimately depends on the image. But might other known works have cut themselves loose from their pictorial beginnings?

I pulled out a festive plum. Larkin reports in a Christmas card: “It’s about the nearest I could get to anything I like - cold outside, warmth & booze & smoke within.” The card shows a 17th-century interior by David Teniers the Younger. A man staggers to the door. Others drink and smoke. What looks like a ham hangs from the rafters. A game is under way, giving it its name, The Card Players. As I know from sniggering sixth-form days, it’s a title shared with one of Larkin’s scatological triumphs. Turning to the poem, it’s clear that, if not a literary facsimile of Teniers, it paints a close picture. Jan van Hogspeuw, Dirk Dogstoerd and Old Prijck, it seems to me, began their mucky lives on canvas.

There was more to come, closer to home. Among the finest of the library’s 10,000 medieval manuscripts is Bodleian MS. Auct D. inf. 2. 11, a Book of Hours illuminated in France for the English market by the Fastolf Master, c1440-50. This scholarly précis appears on a series of 12 postcards published by the library in 1979, each reproducing lavish illuminations from the book’s calendar pages, showing various occupations of the month and associated signs of the zodiac. It was, I think, from between an outsized cat signed “GF Pussy” and a birthday card featuring Margaret Thatcher - “She’s something to be grateful for” - that I caught a gold flash of intricate marginal decoration.

Larkin is one of the few members of the public given privileged access to the Bodleian’s labyrinthine book stacks. He burrowed away here while a visiting fellow at All Souls, for his Oxford Book of 20th-Century English Verse. It must have been during a trip down Oxford’s memory lanes (he read English at St John’s) that he visited our shop and purchased, as further digging revealed, at least seven cards from the calendar series. Each, sent at an appropriate time of the year, alludes to the occupations depicted. December is “not a nice month”, the RSPCA life member writes on the reverse of a slaughtered calf. “What you will be doing at the weekend,” he notes for November - pigs being fed, alongside Sagittarius with bow drawn - “and listening to The Archers too, perhaps.” Larkin enjoyed Ambridge and eating in equal measure.

Finding July was the thing for me. A man reaps wheat in a field bordered by vivid green. Above him burns an intense vermillion sky, laced with leaping tongues of gold (real gold on the real thing). To the right, separated by the master’s ruled margin and Leicester city’s postmark, is a magnificent flourishing lion: the sign of Leo, Larkin’s sign. And on the back? Carefully written out is the poet’s lyric translation of the scene. There’s the searing sky bringing its hammer of heat; the results of what was once sown; the fully leaved field; and, of course, the bounding lion, with his blazing mane, bringing long day into night.

The manuscript is titled 1982, rather than the more familiar Long Lion Days, from Anthony Thwaite’s edition of the Collected Poems, which publishes the only other known copy of the poem, dated July 21, 1982. That version, now kept at Hull, is otherwise identical and presumably a personal record of what, on the evidence of the additional Humberside postmark, he popped in the post that evening or the following day. The signature takes a pop at the ecstatic, primal style of Ted “the Incredible Hulk” Hughes.

I have a hunch that there’s more to be said of Larkin’s painterly eye, and wonder whether, by rereading the poems, we might build up a gallery of visual inspirations. Was he thinking of Louis Lassalle’s Winter (another Christmas card) when he wrote Gathering Wood? Do the strikingly descriptive Dublinesque, Cut Grass or Home Is So Sad hang somewhere? There’s nothing further in this archive to suggest it, but I can almost see them in their frames.

So, back to that unfashionable question: how do poems such as Long Lion Days come about? Like art itself, Larkin’s work is not, of course, just about imitation. It’s true that the immediate fuse to this particular bomb blast of late poetic power was a humble picture postcard. But it formed part of a touching, homely, calendrical correspondence with the most important woman in his life; one that would have whispered with significance in its evocation of the Bodleian, whose hushed spaces they had unknowingly shared. A magical manuscript, indeed, which, in all its rich, coincidental associations – not least its journey home to permanent preservation here - might prove a truth Larkin could never quite bring himself to believe, but which, surely, after all, lies at the heart of every creative act. That “what will survive of us is love”.

Philip Larkin material: © The Estate of Philip Larkin 2008. An edition of Larkin’s letters to Monica Jones, edited by Anthony Thwaite, is being prepared for publication by Faber and Faber, in association with the Bodleian Library (www.bodley.ox.ac.uk ). Christopher Fletcher is head of western manuscripts at the Bodleian Library, Oxford

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