You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!

This is a HaBO from Laura Florand, who wants to be reunited with a book from her teens:
It’s actually a book I really liked as a teenager, a Gothic along the lines of Mary Stewart.
The author uses E.A. Robinson’s “Richard Cory” poem to start the book and references it throughout. The main plot, as I remember it, is that the heroine ends up on a small private island, working perhaps as the secretary or assistant to a retired famous actor. He, of course, ends up being the villain of the piece, but before we find that out, much doubt is cast on the famous actor’s sexy son, who of course ends up being the good guy in the end, trying to protect the heroine from his murderously insane father. (Your classic Gothic plot, really.)
She was, incidentally, a gymnast in school, which helps her greatly in her escapes from the evil villain. (Because we all know how, if you were a gymnast in school, it sticks with you forever and will save your life ten years of unexercised muscles later.) That’s a fairly minor element to her character until she’s swinging from chains or something as she escapes the villain’s murderous plans. Or perhaps misguidedly escapes the hero’s efforts to keep her safe, which she misreads as villainous because, per your usual Gothic romance, no one can ever come out and tell the heroine what is actually going on?
That may not be much to go on, but the “Richard Cory” use always stuck with me, so perhaps someone will recognize it?
It’s so frustrating to me that I can’t remember the author’s name! If you can help out, that would be much appreciated.
A Gothic romance with “Richard Cory.” I’m almost positive someone knows this! I also know quite a few ladies who would be interested in this one.

Bollocks. I absolutely read this, and the only thing I recall is the use of the poem. Is it helpful to confirm that this is a thing that exists? Because that’s all I got. You’re welcome!
Also there was another novel, no idea if it was the same author, that uses Amaryllis.
I feel like I read this once? I know I would have been all over this book in my younger years. Who am I kidding, I’d read it now. It sounds pretty old school–like a Phyllis Whitney but beyond that I don’t know.
Don’t know, but thanks for the earworm: https://www.youtube.com/watch?v=euuCiSY0qYs
Well, at least one other person remembers it. 🙂 So at least I’m not crazy! The only other thing I can remember is that I’m pretty sure the hero had blond hair and green eyes…and it was in some of the first ranks of shelves at the library, so the author must have a last name in the lower half of the alphabet. Not much to go on, alas.
There was a Madeleine Brent with a gymnast – Stranger at Wildings – but the rest doesn’t sound like it matches.
#1) Oh gosh, Laura Florand <<<<>>>>> love your books. #2) so wish I could help you out because #3) I love Richard Corey and #4) I love those Gothic novels and #5) looking forward to finding out the name and author.
Could it be Carnavaron’s Castle by Jean Francis Webb? Here’s the description from Fiction DB (alas no mention of gymnastics):
Determined to write about the legendary actor Charles Carnavaron, Jennie wrangled a invitation to his desolate island stronghold — where no outsider had set foot in twenty years.
Here she disturbingly found both Carnavaron’s widow and mistress living companionably together — but which woman was the mother of Heath, his son and exact handsome double? Heath frightened Jenny more than once into thinking her was the actor’s ghost. But was there really a ghost — and was that the bizarre secret they all tried to hide?
Oh my god, I think that’s it.
Yes! Yes! Manda got it! I recognized it as soon as you said the title. So I was wrong about the author’s name being in the “lower alphabet” stacks of the library.
I’m very happy now. Thank you for the help!
And Bennett, I love Madeleine Brent. Her (his) heroines were always so strong, with such adventurous and different pasts. Stranger at Wildings was excellent.
And thank you very much, Gloriamarie. 🙂 I’m glad you enjoy the books.
Thanks so much to all of you for all your help trying to think of it!
Yay! I’m so glad that was it!!!