Throughout history, famous men and women have expressed their undying love in handwritten letters. In today’s hi-speed age of texting, email and uncertain delivery, not many people take the time and trouble to write a letter; buying a greetings card that says it for you is easier, after all. But if anything is preserved at the back of a desk drawer, it will probably be the handwritten love letter that once upon a time sent someone’s heart shuddering.

Letters of all kinds, but especially those revealing raw emotions such as joyous or unrequited love, are gold dust for biographers. They are often more important as evidence than diaries, which may have been written with one eye to posterity. A letter bares the soul to one other person. It is a private communication not usually meant for anyone else’s eyes. How lucky, then, that so many of us are nosy voyeurs, with few inhibitions about reading the intimate, often painful, letters of others if they come into our hands.

“What makes them compelling and imbues them with a magical quality is the emotional bond between sender and recipient,” says Andrea Clarke, Curator of Early Modern Historical Manuscripts at the British Library, who looks at other people’s letters every day. She has now put together an edited collection of these, published as Love Letters:

2000 years of Romance. The catalyst for the project was working on the archive of the relatively unknown Yorkshire poet and playwright, Gordon Bottomley, who became an invalid with tuberculosis aged 18.

Bottomley wrote many poems and plays, hoping to revive Elizabethan and Jacobean verse dramas, but he never achieved commercial success.

“When I joined the Library twelve years ago, the first archive I worked on was his,” Dr Clarke explains. “He was a sick man, incapacitated by crippling ill health for most of his life, who poured out his heart in hundreds of love letters to the artist Emily Burton.

They touched me deeply and made me wish I had met him.”

In a letter written in 1899, Bottomley finally declares his love for Emily, telling her straightforwardly: “I love you: I do not know how to say anything else!” He begs her to burn the letter if she cannot reciprocate his feelings. Luckily for us she could, and the declaration is preserved. The pair married in 1905 after a ten-year courtship. Bottomley died in 1948, one year after his beloved Emily, and his archive is now fully catalogued and available for consultation by British Library readers (though not all the documents in the book can be seen as they are easily damaged by exposure to light and constant handling).

Other highlights in the book include love notes from Anne Boleyn and Henry Vlll to each other found in a Book of Hours. Anne’s couplet is, significantly, written beneath an image of the Annunciation where the Archangel Gabriel is telling the Virgin Mary that she would bear a son. “By association, Anne was telling Henry that she would provide him with the son and heir he so desperately longed for,” explains Clarke.  There is also a love poem by Ted Hughes about Sylvia Plath, his wife who committed suicide in 1963, part of the Birthday  Letters collection of poems written over a twenty five year period in which he addresses Plath directly and charts their troubled relationship. The couple met in 1956 while both were studying in Cambridge where

‘One by one we made the public benches

Sacred to us. What did we talk about?’

There is one of Charlotte Bronte’s love letters, written in French  to a married professor retrieved by his wife from a bin and, amazingly, stitched back together after her husband had torn them up. There is also what is believed to be the oldest valentine in the English Language, a letter written by Margery Brews to John Paston lll in February, 1477 in which she addresses him as “my right well beloved Valentine.”

When writing a biography of Jennie Jerome, American mother of Winston Churchill, I was able to see (if not touch) the love letters between her and Lord Randolph Churchill, second son of the Duke of Marlborough, before they were transferred to microfilm in the archives of Churchill College, Cambridge. These truly helped me to understand Jennie’s unusual personality and drive, as the couple often wrote to each three times a day during their eight months of unofficial engagement when she was in Paris and he at Blenheim. While their parents were arguing about the size of her dowry, the young couple were arguing about how much they loved each other. Though many of the letters had corners cut off, some decorated corners remain with an arrow that led to a large X, signifying a kiss. If torn off, the corner was carried around in a wallet close to the heart. The self-assured Miss Jerome peppered her letters with French phrases yet, amid the frequent declarations of love, she was not afraid to tell Lord Randolph what she expected from their married life together. “Il faut vous preparer pour une vie de sacrifice,” she warned him if he wanted to marry her.

More recently, while researching my biography of Wallis Simpson, Duchess of Windsor, I discovered a hitherto unknown and unpublished cache of love letters of a rather different kind. These were not addressed to the man she was about to marry, the ex-King Edward Vlll, later Duke of Windsor. Amazingly, these tender letters were written to the man she was in the midst of divorcing, her second husband Ernest Simpson. They make plain her deep seated love for him and her recognition that, by marrying the Duke, she had lost something precious. She and Ernest often described the love that they had together as “congenial” – not a word usually associated with passionate love today. “I can’t believe that such a thing could have happened to two people who got along so well,” she admitted in one.

“Wherever you are you can be sure that never a day goes by without some hours’ thought of you and for you and again in my eanum prayers at night,” she said in another, written more than a year after she had married Edward.

Most bizarre of all in my experience are the love letters of Enid Bagnold, author of National Velvet and the subject of my first biography. Bagnold kept everything, even pencilled missives to her husband Sir Roderick Jones, Chairman of Reuters, suggesting when he might come and visit her – they slept in separate bedrooms – in order to create a new baby. The Joneses produced four children and preserved almost all of the messages between them, however seemingly trivial.

Once when shopping at Fortnum and Mason for a wedding present for someone else, she sent Roderick a love note on a scrap of paper, preserved in the Reuters’ Archive, telling him that the activity reminded her of how much she loved him still, in spite of all the vicissitudes of married life.

Clarke believes that a typed memo or email can never convey the same texture and amount of information that comes from a handwritten letter. “They contain layers of information and reveal much more about a person through the handwriting style, the shape it makes on the paper, as well as the signature itself often with an array of doodles or drawings. Emails are functional and have a place, but they are written in a hurry, not carefully crafted like these letters, which have lots of thought put into them.” It is not only that the impact of the words seems magnified when written by hand but over time, the letters sometimes acquire smells as they may have coffee or tea stains, or be spattered by tears or mud, adding enormously to their power.

Many of the letters Clarke has chosen for her book were written on the eve of battle, or in the midst of war, an especially poignant time for declaring love, or just before a reunion. As the poet Rupert Brooke put it when writing to the actress, Cathleen Nesbitt, “my heart goes knocking when I think of (seeing you again)” and “I will kiss you till I kill you”.

Brooke had been immediately captivated by Cathleen after watching her on stage at the Savoy Theatre. He worshipped her great beauty and bombarded her with marriage proposals. Among his 82 love letters held by the British Library is one from “off Gallipoli”, where he was on his way to fight in the disastrous Dardanelles Campaign. In this note, dated March 18, 1915, he wrote with devastating prescience: “Oh my dear, Life is a very good thing. Thank God I met you. Be happy and be good. You have been good to me. Goodbye, dearest child, Rupert.” He died tragically young from blood poisoning, aged 28, on April 23, 1915.

Clarke has puzzled as to why more love letters appear to have been written by men than women. One explanation is that, historically, men have been better educated and  considered themselves the decision makers. It would not always have appeared seemly for a woman, whose role was to be the passive recipient of a man’s love, to make amorous declarations herself. But, as Dr Clarke concludes: “Perhaps it is simply that women are better at preserving things.”

Anne Sebba and Dr Andrea Clarke will be in conversation at the British Library at 2.30pm this afternoon. (EDS Saturday Feb 1th)  Tickets can be bought at the door.

Love Letters: 2000 Years of Romance, Ed. Andrea Clarke (British Library publications, £7) That Woman – The Life of Wallis Simpson, Duchess of Windsor by Anne Sebba (Phoenix, £7.99)