It was one of the great love stories of the “flower power” era of the 1960s. And when Leonard Cohen met Marianne Ihlen it led to the flowering of the talent of one of the greatest songwriters of the 20th century.

Cohen was a struggling poet when they first came together on the Greek island of Hydra in 1960. And Ihlen became his greatest muse, inspiring one of his emblematic songs, So Long, Marianne, and several others, including Bird on the Wire, while at the same time giving him the confidence to sing in public.

Now, almost 60 years on, an archive of 50 love letters from Cohen to Ihlen, most of them never seen publicly before, are to be auctioned next month by Christie’s in New York under the title Write Me and Tell Me Your Heart: Leonard Cohen’s Letters to Marianne.

Postmarked Hydra, Montreal, New York, Tel Aviv and Havana, and written during pivotal years in Cohen’s career, “these poetic letters brim with both biographical detail and raw emotion”, according to Christie’s, “documenting one of the most captivating love affairs of its time as well as the transformation of a young man into a great artist”.

Highlights from the sale, to run online from 5-13 June, include a letter written in Tel Aviv in September 1960 at the start of their relationship where Cohen says: “It’s hard to write you. The surf is too loud. The beach is too crowded, and you’re too much in my heart to put anything down.” The letter is expected to fetch up to $9,000.

Marianne Ihlen leading Leonard Cohen and friends on donkeys along a stone path in Hydra, in 1960. Photograph: James Burke/The LIFE Picture Collection/Getty Images

One of the coveted items also for sale is a cracked bronze bell from the couple’s home on Hydra that Cohen famously included in the chorus of his haunting song Anthem from the seminal album The Future in 1992.

Also on offer is a letter from New York after his first major performance in the city in February 1967. “Every singer you’ve ever heard of was there performing. Judy Collins introduced me to the audience, over 3,000 people, and they seemed to know who I was, mostly because of [his song] Suzanne.”

On their relationship, Cohen notes in one letter that he’s “always thinking” of Ihlen and her beauty: “What can I say? You have gone deep inside of me. I want to sing when I remember all our work of love.”

In another letter he writes to “Darling Mu” from a snowy Montreal: “Right now I’m thinking of a certain night when we walked along the Rue des Ecoles. You put your face against my arm, held me tight, and closed your eyes, and let me see for the both of us. I have never felt so moved by an act of trust.”

The moment he describes conjures up a line that would emerge later in So Long, Marianne: “You held on to me like I was a crucifix / As we went kneeling through the dark.”

So Long, Marianne appeared on Cohen’s first album, Songs of Leonard Cohen, and the song featured in his concert repertoire throughout his career. And towards the end it became a singalong with the audience.

The back sleeve of his second album, Songs from a Room, features a photograph of her at Cohen’s typewriter, draped in a white towel in their home in Hydra. After leaving the island, they lived together in Montreal for a while but Cohen drifted off into the arms of other women including stars such as Joni Mitchell and Janis Joplin.

Marianne Ihlen, left, holds her son, Axel Jensen Jr, and Leonard Cohen, second left, with friends in Hydra, Greece, in October 1960. Photograph: James Burke/Time & Life Pictures/Getty Image

When he made his British debut in the middle of the night at the Isle of Wight festival in 1970, he introduced So Long, Marianne with the following plaintive words: “I wrote this for Marianne. I hope she’s here; maybe she’s here. I hope she’s here. Marianne …”

Shortly before she died at 81, he was still reaching out to her. He wrote to her on her deathbed in her native Norway in July 2016: “Well Marianne, it’s come to this time when we are really so old and our bodies are falling apart and I think I will follow you very soon. Know that I am so close behind you that if you stretch out your hand, I think you can reach mine.

“And you know that I’ve always loved you for your beauty and your wisdom, but I don’t need to say anything more about that because you know all about that. But now, I just want to wish you a very good journey. Goodbye old friend. Endless love, see you down the road.”

Cohen was indeed very close behind her. He died four months later aged 82.