The secret love life of one of the leading female British painters of the 20th century has been revealed in a series of love letters to the wife of a Glasgow sheriff.

Joan Eardley – whose art is famed for its depiction of children playing in rundown Glasgow tenements as well as the fishing village of Catterline on the north east coast – wrote daily letters to Lady Audrey Walker, with whom she was in a close relationship from 1952 to her death in 1963.

In a new book, published 50 years after the artist's death, Glasgow-based author Christopher Andreae publishes a selection of letters from Eardley to Lady Walker for the first time since a family embargo was lifted in 2009. The letters not only reveal the affectionate and loving side of the artist, and her daily artistic routine, but also makes plain her sexuality.

In one, from 1955, the artist wrote: "I've had a good day and perfection would be to have to you here tonight. But that's a thing I daren't let myself think about. The important thing is that you are all right."

In another, from the same year, she penned: "I just feel I love you so much and there just ain't words to say it - not words that mean what I feel inside."

Mr Andreae said: "Joan met Audrey in 1952 in Glasgow and when she settled in Catterline, she wrote her a daily letter because, it is clear, she was in love with her.

"They are love letters, but they also address what life was like for Joan in Catterline and what she did there.

"Audrey's son told me she used to visit Joan 'frequently'. Now the letters are in the National Library of Scotland and in the public domain."

The letters also explain the challenges and joys of painting the wild seas of Catterline, the small fishing village near Stonehaven where she lived and worked after studying at the Glasgow School of Art.

Born in Sussex in 1921, Eardley moved to Glasgow in 1940 and, after her studies, had a studio in the Townhead area, where she saw and painted many deprived children. Her career was cut short in 1963, when she died of cancer at the age of 42.

The author said: "The point of the letters is the openness about her sexuality. From that point of view, it is good these things should be out in the open."

No letters to the artist from Lady Walker – wife of Allan Grierson Walker, Sheriff Principal of Lanarkshire – survive. It is believed they were accidentally disposed of after Eardley's death. However, in a tribute to the artist written by Lady Walker and published for the first time in the new book, she compares the artist to the North Sea in all its different moods.

Lady Walker wrote that the summer sea was the Joan "everyone knew" but the winter sea in "its indomitable grandeur and the wild, turbulent and terrifying splendour" was "Joan too".

Mr Andreae said: "That perhaps explains why Joan Eardley was inspired by the sea. It says in the letters how she would be drawn to the sea and couldn't pull herself away and became so caught up in it."

The letters, Mr Andreae added, not only cast light on the artist's hitherto unconfirmed private life but also her life and art.

The revelations come as a major new exhibition of the artist's work opens at the Scottish Gallery in Edinburgh, which will run from April 3 to 27.

l Joan Eardley, by Christopher Andreae, is published by Lund Humphries.