I've been a fan of Donoghue's academic works on the history of same-sex relations between women, but although I've collected up a number of her novels, I've only recently decided to prioritize them on my reading list. One essential thing to know, going in, is that a Donoghue novel about romantic and/or sexual relationships between women in history is not a "lesbian historical romance." These aren't formulaic books with happily-ever-after endings, they're fictionalizations of the lives of real historic women. Messy, complicated lives that don't resolve easily into feel-good endings. But neither are they necessarily tragedies.
The Sealed Letter interprets the life of Emily Faithful (nicknamed "Fido"), a 19th century English feminist, writer, and printer, who became tangled up in the scandalous divorce trial of a friend, alternately being accused of abetting the woman's infidelity and--by extremely veiled suggestion--of being part of the woman's infidelities. The setting of the story explores the precarious lives and careers of women who tried to expand the options for women in all fields of life, while having to dodge accusations of being "unwomanly" for doing so and struggling for the necessary financial support. There is also a great deal of exposition regarding divorce law in England at the time.
All this necessary exposition sometimes teeters on the edge between presenting historic research in the guise of fiction and providing a fictional story with the necessary background for the reader. I found it to keep the right balance, but as a fan of historical research I may have a fairly high tolerance. To some extent the story works best as a mystery: doling out clues to how the present state of affairs came to be through the lens of an entire cast of unreliable narrators. (I don't think there's a single viewpoint character who is entirely honest with the reader.) This unreliability delivers a delightful payload at the very end of the story when we're treated to one last tidbit about Fido and her divorcing friend that throws the puzzle pieces up in the air and leaves them to settle in an entirely new configuration.
Donoghue is a skilled and polished writer and managed to pull off a technique that many writers would struggle with. The book is written in multiple first-person present-tense viewpoints. I really stumbled over the present tense aspect in the first several pages because it left me confused and uncertain exactly how the people and events that were being discussed were related to each other. But after needing to re-read the opening several times to settle in, this aspect of the writing style faded to invisibility. The shifting first person approach worked excellently to show the skewed and filtered understanding that each character had of reality, allowing the reader to build their own understanding of what might have happened. I thought this worked particularly well given how the story is based on actual fact--but a set of facts that are themselves incomplete and ambiguous.