Simon the Coldheart is one of Georgette Heyer’s few non-Regency romances, set instead during the Hundred Years’ War. The Open Road-published version of this has dropped to $0.99 at Kindle and Kobo US right now, and it’s discountable at Kobo. Note that there seem to be several versions of this, so make sure you get the one on sale.
The multiple versions thing often means it’s in the public domain somewhere. But I’ve checked Project Gutenberg, and it doesn’t show up there. I also checked Faded Page and PG Canada, and it doesn’t show up there either. So I’m uncertain as to why the multiple versions, but since this particular one is Open Road, which is a reputable publisher, it should be okay.