In the beginning, long before Joan Bakewell and Helen Mirren, the thinking man's crumpet was Lady Antonia Fraser. Tory grandees swooned before her, avant-garde playwrights fell at her feet, Cromwellians and Catholic prelates alike murmured her praises. The biographer of Boudicca, Marie Antoinette and the entire harem of Henry VIII now turns her formidable intellect to love, in perfect time for Valentine's Day.

This neat volume was originally published 25 years ago and has been revamped once already, in 1989. Diligence rewarded three times, perhaps, though I'd have preferred her 1975 selection of love poems. Pithy elegance or the well-chosen word are not the main talents here; when the letter-writing talent flourished, back in the days before e-mails and the telephone, writers spent hours churning out many pages, as Dora Carrington admits. One hopes they were not quite so verbose when the loved one was actually in their arms.

HeloIse and Abelard are a case in point. He comes across as a precious old bore, quoting chunks of Testament at her. She is passionate, even saucy, with a remarkably modern tone: "When you appeared in public, who did not hurry to catch a glimpse of you . . . ? Every wife, every young girl desired you in absence and was on fire in your presence; queens and great ladies envied me my joys and my bed." Hot stuff indeed. I wonder how she managed to smuggle such vivid prose past her abbess?

I preferred the reticence of Lucrezia Borgia, writing to "My dearest Missa Pietro" the court poet, even though, to today's ear, it sounds more like a brush-off than a come-on: "I decided to put off writing to you . . . In your letters you express with such ease all that you feel for me, but, I, just because I feel so well-disposed towards you, am unable to do so." Still, she signs it: "Your own Duchess of Ferrara", so maybe the real message is in the salutations.

The men of action are commendably crisp. Bonaparte summons Josephine in 1797: "I send you a thousand kisses. I am in bed." Oliver Cromwell gently tells his wife: "Thou art dearer to me than any creature" - a choice of words echoed 200 years later by Thomas Carlyle. Jane Carlyle's letter witters on about her "bowel complaint" - after her death, according to Fraser, her husband was shocked to discover how unhappy she had been much of her life, but judging from the slight evidence here, she was probably a bit of a trial.

Diderot, on the other hand, entertained his mistress Sophie Volland by tossing off acres of entertaining verbiage to her, mostly about the dinners and soirees he attended in her absence. He teases her: "So you think that I really love you? And what makes you think this, pray?" He does seem to have enjoyed himself hugely in the company of this or that fashionable baroness. If I'd been on the receiving end, he might have had some tart replies, or a rolling pin about the ears.

Phrases leap out. "Why do people hate me?" wails the Czarina Alexandra to her husband in December 1916. Poor, mad Zelda Fitzgerald calls Scott, the author of The Great Gatsby, "Goofo" and begs: "Come Quick - Come Quick to me - I could never do without you." Lady Shigenari, wife of the governor of Nagato, wishes him well as he goes off to his last battle; she tells him that she has "abandoned all hope about our future together in this world", and has resolved to "take the ultimate step" so that she can be "waiting for you at the end of what they call the road to death". She did it, too. This is love, samurai-style: it's a funny old world.

Complaints about the post abound. It must have been frustrating when there were no other means of communication, when separation could mean months or even years of wordless longing. How easily we take for granted the phone, the three-word note attached to an Interflora bouquet, the scribbled message - "Guess who?" - in those manufactured cards. Nobody is going to make a collection of those. But in this volume are plenty of examples of how it's done, when lovers care to do it well.

Edwina Currie's most recent book is This Honourable House (Little, Brown)