One of the country’s best-loved children’s authors has published his first book for grown-ups in 40 years.

Sir Michael Morpurgo, who spent time living and working in Canterbury in his youth, is celebrating the season with his latest non-fiction book, Spring.

After going to the GP with what he thought was indigestion last year, the bestselling author found himself undergoing a triple heart bypass – and an enforced break from writing.

“We all have this cliché that you’ve got pains in your chest and it’s agony. Oh no, I went to my wonderful NHS doctor in Hatherleigh in Devon and I said, ‘Look I think I’m getting indigestion rather too often and it’s rather strange because it always comes in the same place between my shoulder blades.

“Within 10 days I was having it done because it was found all three arteries were blocked and they wouldn’t let me out of hospital because it was too serious,” the 81-year-old former children’s laureate and bestselling author of War Horse recalls.

When he left hospital, he found he had some trouble getting back into writing.

“I couldn’t really concentrate because after that my mind had gone somewhere else, but it’s gathered itself now, mostly. An enforced break wasn’t really good for me. It lowered my spirits not to be writing. Now, I’ve always got a project and I like that.”

Michael’s bestselling books, including Private Peaceful and Kensuke’s Kingdom, frequently introduce children to harder themes while familiarising them with the beauty of the natural world.

His latest, Spring, his first adult non-fiction in 40 years, written before his heart problems were diagnosed, is a celebration of the season that he loves, the anticipation, the signs it is upon us and his memories of the years at his farm in Devon with his wife, Clare, to whom he has been married for 61 years.

Both Michael and Clare are former teachers, with the author teaching at Wickhambreaux Primary School, near Canterbury, after studying at the King's School, an independent school in the city, as a child.

In 1976 the couple founded the charity Farms for City Children which has given more than 100,000 children from disadvantaged communities the experience of working together on their three farms in the heart of the British countryside.

Readers can gently meander with Michael in his beloved Devon as he wonders at the nature, the insects, frogspawn-spotting, the Torridge River, the lambing season, the birds, the first bluebell of spring and the return of swallows.

The landscape of the countryside may not have changed superficially, he says, but it has changed.

“We [society] have overfarmed the land, just as we’ve overused every inch of the land that we’ve got. We pollute it too much, we try to extract too much out of it.

“When we came to live in Devon over 50 years ago, the rivers weren’t perfect but there were trout in the streams, but there’s hardly any now. People are working hard to put that right but we have to have the next generation of children understanding that we’ve been greedy for our food and pleasure.

“We have to understand that we do damage when we take out cars too much or fly on planes too much. We have to get to a situation where we are more sensitive to the needs of the world about us.”

Spring is also a reflective read, a gentle nostalgic step back in time to his life and his observations about how some things change and others don’t. The book is part of a series on the seasons – Bernardine Evaristo will tackle summer, Kate Moss, autumn and Val McDermid, winter.

Michael is optimistic about the future because of education, he says, and feels that the next generation will put the environment right.

He still visits schools and book festivals, while films and plays of his stories remain popular, the latest of which, White Horse, is based on his book The White Horse of Zennor and Other Stories. It all helps him keep connected with young people, children and teachers.

Meanwhile, although he is back to writing he says he is still recovering.

“I’m much more muddled than I used to be, or maybe that is just age catching up.”

Spring: The Story of a Season by Michael Morpurgo is published by Hodder Press, priced £16.99, and is available now.