Cassandra Brown is a young physicist working on a "4-D accelerator" at a lab in London, when there is an explosion, and all of a sudden the lab looks very different, and there's a strangely dressed man. Of course, no surprise, she's in 1815, at Font Hall, the seat of Harry, Lord Font. Harry is an experimenter himself, and somehow Cassandra's experiment interacted with Harry's fooling with electricity, and she ended up back in time.
I love this book; I've read it several times, but still enjoy it. It ticks multiple boxes for me: time-travel, Regency, romance, a little bit of mystery, and even some genealogy! Imagine if you, a college-educated late-twentieth-century woman were suddenly whisked back to Regency England, with all its strictures on just what women could and could not do! How would you handle it? Dr. Cassandra Brown finds herself in just that position, facing Harry, Lord Font in his stately home, where, in her time, she has her laboratory. Together, armed with only Cassandra's calculator and Harry's slide rule they begin the calculation necessary to send her home. But in the meantime, Harry has promised his young sister-in-law to visit London in search of a bride, and Cassandra accompanies him. Very enjoyable; something of a romp, but full of interesting characters (including Jane Austen and her niece, briefly). Recommended.