Baudelaire introduced ordinary objects into poetry – likening the sky to a pan lid – and by doing so revolutionised poetic language. Likewise, Seamus Heaney introduced Northern Irish vernacular into the English lyric, peppering his lines with words like glarry, the Ulster word for muddy; kesh, from Irish ceis, a wickerwork causeway; and dailigone, “daylight gone” or dusk, from Ulster-Scots. It is this that gives his writing a mulchy richness and cultural resonance that remain unique in contemporary poetry. One of the key poems in North (1975) is a version of Baudelaire’s The Digging Skeleton, to which Heaney brings an Irish flavour – the skeletons dig the earth “like navvies”. It’s especially rich as digging for Heaney is also a metaphor for writing, while the archaeological metaphor resonates with the darkly symbolic bog poems.

Bringing all Heaney’s poems together in one volume, this collection lets us see for the first time all the archaeological layers that make up his oeuvre, from the talismanic Death of a Naturalist (1966) to the visionary long poem Station Island (1984), on to the parables of The Haw Lantern (1987) and the intimacies of The Human Chain (2010), the last volume published during the poet’s lifetime. A key poem in that collection, Chanson d’Aventure, describes his journey to hospital in an ambulance following a stroke: “Strapped on, wheeled out, forklifted, locked / In position for the drive”. The book also makes available at last Heaney’s prose poems, Stations (1975), released in a small press edition by Ulsterman Publications, which Heaney effectively kept under wraps as he felt the publication of Geoffrey Hill’s Mercian Hymns – “a work of complete authority” – had stolen his thunder in this form.

The editors have taken the admirable decision to leave the published volumes intact, so that their careful ordering, something Heaney learned from Yeats, remains in place. Between each volume they insert all the contemporaneous poems that Heaney published in magazines and in pamphlets, as well as a selection of previously unpublished manuscript poems. There are a surprising number of these uncollected poems, and what surprises too is the richness of these previously undisclosed seams. In the earliest poems we see the influence of Ted Hughes and Gerard Manley Hopkins as Heaney gropes his way towards his distinctive personal style. There are important occasional poems such as An Open Letter, protesting about his inclusion in the 1982 Penguin Book of Contemporary British Poetry, and a smattering of villanelles that simmer deliciously without reaching the intensity of the collected verses. Elsewhere we see Heaney trying out different metaphors for the act of writing, such as casting a line, before he settles on the metaphor of digging in Death of a Naturalist. And there is the occasional radiant gem, such as Aran: “The rock breaks out like bone from a skinned elbow / And the island coughs itself into high cliffs / That drop straight down to goose-flesh waves on the winter sea.”

What impresses is the consistency and the variety, coupled with the continuing ability to surprise us, for while Heaney always hugs the shore of tradition, he is not afraid to experiment, developing and deepening his craft. In an interview with Dennis O’Driscoll, discussing the attention he pays to the sounds of words in Wintering Out (1972), Heaney was dismissive when asked about the sound poets Kurt Schwitters and Bob Cobbing, pronouncing: “I have nothing at all to declare in that area.” Yet when I corresponded with Heaney in 2006 about experiments in French poetry, including the formal innovations of the Oulipo, he responded with enthusiasm. The French tradition of syllabic poetry, where syllable takes precedence over stress patterns, informs the slithering six-syllable lines of Beyond Sargasso, one of many poems about eels, in Door into the Dark (1969). Heaney’s love of a pun is pushed to the limit with chilling effect in Two Lorries, his only sestina, which reinvents the form by metamorphosing the end word “load” into “lode”, “lead”, “payload”, and finally “explode”. In Seeing Things (1991) Heaney creates a form of his own invention in the clarifying 12-liners of the sequence Squarings, a form he was to return to again and again, where he attains a new simplicity and transparency, almost stripped of rhetoric, as the skin is stripped from an eel: “Roof it again. Batten down. Dig in. / Drink out of tin. Know the scullery cold, / A latch, a door-bar, forged tongs and a grate.”

Some of Heaney’s boldest work is to be found in his translations, and while these, including his prizewinning Beowulf, have mainly been published in a separate volume, we find the freer translations collected here, many of which come close to Alice Oswald’s idea of “trans-shifting”, as in his reworking of Dante in Station Island. Two poems in Human Chain riff on Virgil’s Aeneid VI, where Virgil visits the underworld. The Riverbank Field moves Virgil across time and place, replacing the Lethe with local river the Moyola:

Ask me to translate what Loeb gives as
“In a retired vale…a sequestered grove”
And I’ll confound the Lethe in Moyola

By coming through Back Park down from Grove Hill
Across Long Rigs on to the riverbank –
Which way, by happy chance, will take me past

The domos placidas, “those peaceful homes”
Of Upper Broagh. Moths then on evening water
It would have to be, not bees in sunlight …

In Route 110, we see Heaney bag a secondhand Virgil in Belfast’s Smithfield Market, before taking the bus, Cookstown via Toome and Magherafelt, and as we follow him, as in Joyce’s Ulysses, Virgil’s underworld and the shades of the Troubles are superimposed: “And what in the end was there left to bury / Of Mr Lavery, blown up in his own pub / As he bore the primed device and bears it still // Mid-morning towards the sun-admitting door / Of Ashley House?”

This book is a landmark, and while some readers, including this one, may at first balk at 500 pages of notes, these have been compiled with loving expertise, leaving no stone unturned. Above all, the notes let us glimpse Heaney’s processes – “all those anglings, aimings, feints and squints”, as he puts it in Seeing Things, with reference to the game of marbles – how he tries out ideas in draft, sometimes abandoning them, and then returns to the same material later, before arriving at the final poem. The resulting volume lets us see Heaney’s work, whose ripples we are still learning to navigate, for the colossal achievement it is, and it reminds us that Heaney is not only a keeper but an enricher of the word-hoard.

• Philip Terry’s latest collection is Dante’s Purgatorio (Carcanet). The Poems of Seamus Heaney, edited by Rosie Lavan and Bernard O’Donoghue with Matthew Hollis, is published by Faber (£40). To support the Guardian, order your copy at guardianbookshop.com. Delivery charges may apply.