Frances Harris, who has died aged 71, made her name as the author of three remarkable studies of leading figures in late Stuart Britain. Her first book, a biography of Sarah, Duchess of Marlborough (1991), announced to the world the arrival of a significant historian as well as a fine writer. Determined that Sarah should be viewed as a major political figure in her own right rather than just a wife and a favourite, Frances insisted, somewhat to the publisher’s consternation, that the book should be titled A Passion for Government, even if forced to admit that the duchess herself often displayed too much passion and too little government.

It is little surprise that Frances, as one of a rising generation of feminist historians, was attracted to such a figure, a woman who had no difficulty in making her way in a man’s world. For her source material she was able to make full use of the Blenheim papers in the British Library which she herself helped catalogue. She had joined the British Library’s department of manuscripts, where we were colleagues for more than 20 years, in 1979. Given her specialist period, she was fortunate in the timing of her appointment. The library’s acquisition of the Marlboroughs’ papers was quickly followed by the arrival of a rich succession of other 17th- and 18th-century archives.

It was the library’s purchase of the Evelyn papers in 1995 that inspired what is almost certainly Frances’s finest book, Transformations of Love (2002). The friendship of the diarist John Evelyn with the much younger Margaret Blagge, a maid of honour at court and subsequent wife of Sidney Godolphin, had in the 1950s been interpreted in crude Freudian terms as a predominantly sexual infatuation. Though this approach had been discredited, it fell to Frances to describe the theological context in which Blagge and Evelyn sought, through their friendship, to find spiritual fulfilment and to untangle a relationship familiar to their contemporaries but quite alien to the modern world.

Frances was only too aware that feminist history had in the past not been comfortable in dealing with women’s religious lives. However, quite apart from her intimate knowledge of the period, she was herself well equipped to understand and describe these. Though no longer observant, she had been brought up a strict Anglican and knew the liturgy before she could read. The book was awarded a Wolfson history prize in 2004.

The writing of it had been assisted by an Arts and Humanities Research Board exchange scheme that allowed Frances to spend three months at Royal Holloway, University of London in 2000. It was by no means her only link with the academic world. When Lisa Jardine established the centre for editing lives and letters (Cell) at University College London in 2001, Frances joined the steering group, serving until 2007. She also acted as a co-supervisor of PhDs for the University of Sheffield and as an external examiner for the University of London.

Evelyn was well known in his own lifetime for his garden at Sayes Court and his treatise on trees, Sylva (1664). Consequently, Frances’s work on him led to a growing interest in garden history. She began publishing on the subject and in 2004 she co-curated the British Library exhibition The Writer in the Garden.

Frances retired from the British Library in 2010 as head of modern historical manuscripts. Apart from serving as honorary secretary of the Friends of the National Libraries (2013-18), she was able to concentrate on a further book, The General in Winter (2017). In it she returned to the Churchills to describe the relationship between John Churchill, the first duke of Marlborough, and the lord treasurer, Sidney Godolphin. In an autobiographical essay published in 2012, she had written that two of the themes that most interested her in history were the role of friendship and the impact of “the good person” on the world. It was no accident that “friendship” appeared in the subtitles of two of her books. For her, the close bond between Churchill and Godolphin and the quiet heroism of the latter as he sought to defend his friend against his enemies in Britain were powerful illustrations of these two themes.

Born in Australia, in the foothills of the Dandenong Ranges, south-east of Melbourne, Victoria, Frances was the daughter of Esther (nee Hall), a commercial artist, and Laurence Harris, a civil servant. She grew up in nearby Burwood and attended Methodist Ladies’ College, Melbourne.

In 1967 Frances and her mother travelled to Britain to join her father, who had been posted to the Australian high commission. When her parents returned to Australia in 1971, Frances remained, and studied English at Westfield College, London University, graduating in 1971, and being awarded her PhD in 1975. She went on to take a postgraduate diploma in library and information studies at UCL, and, in 1976, was recruited to join the group which had just begun working at the British Library under Robin Alston on the 18th Century Short Title Catalogue.

Frances ascribed her intellectual awakening to a Christmas present from her parents when she was nine. This was Kipling’s book of short stories for children, Puck of Pook’s Hill. For one who was growing up in “a new house in a new suburb”, it opened her eyes to the fact that there was a place much richer in history than the one she knew and that the past could be as real as the present.

Just before Christmas 2020, she was diagnosed with myeloma. She is survived by her long-term partner, Elfrida Roberts, with whom she entered into a civil partnership shortly before her death, and by her brother, Laurie.

• Frances Marjorie Harris, manuscripts curator and biographer, born 8 January 1950; died 17 February 2021