In this age of instant communication the idea of the midnight letterbox - posting a letter in the dead of night to ensure its early collection in the morning - seems almost quaint.

Edwin Morgan uses the expression only once in more than 500 pages of collected letters. Many of them were doubtless posted under different circumstances, but it is easy to see why 'midnight letterbox' caught the eye of editors James McGonigal and John Coyle. It conjures up an enduring image of Morgan emerging into darkness from the Whittingehame Court flat in Glasgow (his home for 40 years) and dispatching letters to "artists, filmmakers, composers, editors, academics, and readers young and old..."

The first Scots Makar of the modern age didn't quite make it to email, though illness eventually forced him to respond by fax to emails sent to his website. And as we wait dolefully for 'digital legacy' to render letter collections extinct, there is much to be thankful for in Morgan's assiduous preservation of his typed and handwritten correspondence. The material is drawn from Special Collections at Glasgow University Library where it is so abundant that the editors "did not need to prospect further into other literary archives..." With such a cache, the process of selection is of considerable interest but McGonigal and Coyle claim a simple agenda. The "realism of price and production" was a defining issue and they were economical with addresses and salutations to ensure a maximum number of letters. The end result is a "selection of a selection of a selection". Morgan would surely have enjoyed the acronym - SOSOS - and perhaps detected some concrete poetry potential in it.

The letters are arranged by decade following their author's preference for "a decades-eye view". He burned personal letters when he left for the army in 1940 - no explanation offered here - and there are none from his service years, which seems unusual given the importance of letters in the lives of countless other soldiers. Comparatively few letters are presented from the 1950s but already patterns emerge. Each decade has a favoured correspondent and in the 1950s it was Greenock poet WS Graham who had relocated to Cornwall. The two exchanged poems and advice in a continuation, say the editors tantalisingly, of a habit they formed before the war. In one letter Graham has asked Morgan to comment on his poem The Nightfishing and is rewarded with three pages of close analysis. Morgan is unflinching in his criticism but also careful to offset it with due praise, a balancing act common to most of the advice that he proffered over the years.

In the 1960s Morgan departed his parents' house for Whittingehame Court, started a relationship with a man called John Scott, furthered his professional teaching career at Glasgow University and stepped up his interest in poetry translation and innovative form. Letters and recipients mushroomed accordingly but again one stands out. He corresponded regularly with Ian Hamilton Finlay who, encouraged by Morgan, was promoting visual poetry in his periodical 'Poor. Old. Tired. Horse'. The letters to Hamilton Finlay also reveal evidence of Morgan's growing influence in the world of educational and arts bureaucracy. His interview for the chair of English at newly built Stirling University was unsuccessful, but he offers Finlay advice on how to access his new connections there and promote the installation of his poem-sculptures on the university grounds. Stirling as Little Sparta with Edwin Morgan at the English helm is something to savour, but the university commissioned sculptures from the likes of Justin Knowles and later hired Norman MacCaig as Poet in Residence. Morgan, however, continued to promote the interest of other artists. In 1977 he wrote to Scottish Arts Council Literature Director Trevor Royle (better known now as a military historian) asking him to fund recording equipment for Tom Leonard's sound poetry projects. Leonard, he said, had "serious intent and genuine originality" and "was not out to collect some high-grade toys for himself or his friends".

There are few letters here that wouldn't bear highlighting or quoting from. In the 1970s Morgan's publisher, Edinburgh University Press, was showing signs of giving up on poetry completely and one can sense Morgan's relief when Michael Schmidt of Carcanet took an interest in his work. The Morgan to Schmidt letters sparkle with wit and literary allusion, perhaps anticipating the day when they would be published under the recipient's imprint. For all the weight of words and fine expression, however, there is a sense of measure and restraint that pervades many of the letters in the book. It was only in the last part of his life that Morgan really took the brakes off. He made his sexuality public at the age of 70 and wrote to publisher Hamish Whyte in 1999 to say that he was in two relationships ("two loves at my age, isn't it absurd"). One of them is with a young muse called Mark Smith and Morgan's letters to him, unlike many of the earlier ones, are full of joy and abandon.

The final letter selected was dictated in 2010, six weeks before Morgan died. It is to the Edinburgh LGBT Age Project, a helpline for elderly gay people. "The most important thing now," he said, "is to see things clearly and to discuss things openly..." The Scotland that Edwin Morgan died in seems so much better suited to him - personally, politically and culturally - than the one into which he was born. Sadly, to open the doors and begin again is not an option.

John Coyle and James McGonigal will be speaking at Aye Write! on April 17