Start with high-confidence gilded age romance books, then narrow by subgenre or adjacent topics.
A Certain Age
Beatriz Williams
An Heiress to Remember
Maya Rodale
The Last Madam
Chris Wiltz, Christine Wiltz
The Scarlet Sisters
Myra MacPherson
All the Pretty Places
Joy Callaway
The Seamstress of New Orleans
Diane C. McPhail
Gentlemen Prefer Blondes
Anita Loos
North and South
Elizabeth Gaskell, Gemma Whelan
The Fifth Wheel
Olive Higgins Prouty