Georgette Heyer’s Regency stories are marvelous. The dialog is terrific, you will learn a ton of terms (I recommend getting “Regency World” to go along with, as the author references many of the characters in the novels to explain terms like “mantua maker”, “dandy”, ‘Corinthian’, etc.) Interestingly, she also wrote a handful of murder mysteries set in the 1930s which are wonderful little period pieces!
Also good mid 20th century works (which are either ‘modern’ or else “Victoriana”) would be works by Victoria Holt (who also wrote under the name Jean Plaidy a very easy reading but reasonably factual series about most of the kings, and Queens, of England as well as other notable characters). Also good for clean romance would be Mary Stewart (and her Arthurian quartet starting with the Crystal Cave is set in Brittain around AD 400-500); ‘modern 1950s’ novels with a Scottish setting by D. E. Stevenson.
Sally Goldenbaum has about 9 books out (recent 2000 vintage, but clean) based on a group of knitters in Massachusetts, of all ages. Even if you don’t like knitting (and I do), the characters are interesting and the murders are well done. (My 86 year old mother adores these. . .and she doesn’t knit!)
The novels by Miss Read which are set in mid 20th century England are marvelous --start with Village School and Village Diary (She is a school teacher herself). She also writes about a nearby village in a series “Thrush Green”. No sex, no murder, just marvelous characters and descriptions of life that will make you either nostalgia or just wishing to visit there forever.
Josephine Tey for both mid 20th century mystery (A shilling for candles) as well as possibly the best ever 20th century ‘apology’ for Richard III (“The Daughter of Time”) that you will ever read.
Modern: Susan Wittig Albert’s series starting with The Tale of Hill Top Farm which sets Beatrix Potter (yes, that author) as a solver of gentle mysteries and treats the years 1904 to 1913 when, after recovering from the death of her fiance, she moves to the said farm, and meets (and later marries) the solicitor William Heelis. Be warned, the animals ‘talk’ so the fantasy level is high, but again, lots of new terms (Yorkshirisms) and a decent fidelity to the author herself make this a good series.
I was always fond of the American author Gwen Bristow, who wrote a Civil War trilogy as well as novels about women who were part of the 1849 Gold Rush (Calico Palace), the early road west and Texas independence (Jubilee Trail) and the Revolutionary War in Charleston (Celia Garth). Lots of sprawling story.
A fantastic novel by Helen Hunt Jackson and set in the time of the recently canonized Fr. Junipero Serra is “Ramona”.
A slow paced but thought provoking read is George Elliot’s “Middlemarch” --her "Silas Marner’ is quicker and more well known.
I liked Thomas Hardy more when I was younger; his later works are too depressing for me now! However, of those, “Far From the Madding Crowd” is the least depressing!
I liked the early mysteries by Joanna Fluke about baker Hannah Swenson (set in Minnesota) but after about book 7 or 8, they go downhill.
I love Dorothy L Sayers Lord Peter Wimsey novels/stories. A couple of "d-mns’ here and there but otherwise nothing to take exception at. Start with “Whose Body” (the first) and end either with “Busman’s Honeymoon” (which is the last one Ms Sayers wrote as a complete novel, and the two short stories which take Lord Peter to his 'semi retirement) or continue on with “Thrones and Dominations” etc. in which Jill Paton Walsh takes to completion the unfinished works Ms. Sayers left on her death.
Of course there is Sir Arthur Conan Doyle’s Sherlock Holmes series.
And there is also Thomas B. Costain. He wrote not only historical works such as “The Three Edwards”, “The Magnificent Century”, and “The Last Plantagents”, but also historical fiction like “The Silver Chalice” (about the cup Christ used at the Last Supper); “The Black Rose” (from a legend about the father of St. Thomas A Becket); “The Moneyman” (about Jacques Coeur, the great financial genius who pioneered a department store in the 15th century in Paris, and who was persecuted by the king), as well as other interesting stories.
Who could forget Erle Stanley Gardner’s “Perry Mason”? When I was little, they made me want to be a lawyer.
A lot of people know Jane Austen for “Pride and Prejudice” but she wrote five other full length novels as well as many letters and short works (the Juvenalia). My own favorite is Mansfield Park (but be warned, it is most peoples’ ‘least favorite’). I also like “Persuasion” (her last work) but that one is very well liked, as is “Emma”. Sense and Sensibility is too uneven for me (the two sisters representing one “Sense” and the other “Sensibility” are too rigidly drawn!), and Northanger Abbey’s heroine and the concept of her having such a penchant for ‘horrors’ that she not only misreads perfectly normal things as ‘fantasies’ but actually MISSES the REAL horrors she encounters because they are done matter-of-factly make her (to me) too much a caracature of a “Valley Girl”, but there are lots of well-drawn characters and it’s fun to see the similarities between a Mr. Collins in P&P and a Sir Walter Elliot in Persuasion, or a ‘silly mother’ like Mrs. Bennett in P&P compared to equally silly mothers in Persuasion, Sense and Sensibility, as well as domineering parents, ne-er-do-wells, etc.
I also enjoy the Nero Wolfe Novels by Rex Stout. I believe “Fer De Lance” was one of the earliest. The very earliest have the narrator Archie Goodwin coming off more like a young "Gumshoe’ style tough; but very early on his style becomes much more that of the New York everyman. . .and the time line goes from the early 1930s up to the early 1970s, even though the characters themselves do not ‘age’.
Gee I could go on forever, but I’d better stop now!