Name that book

I read a short book, less than 100 pages, and it is driving me insane trying to find it or figure it out. I cannot remember the name or the author. It was a historical romance with a highborn lady, needing to marry, and has a secret lover, with a hidden identity, and they write secret letters to one another. They meet up at a party or something with masks and have several secret encounters before she figures out who it is and I think it is her enemy. Can you name the book because it is driving me crazy and I have googled everything I can think of. PLEASE HELP ME FIGURE OUT THIS BOOK!!!!

I suggest you read the posting guidelines and follow the hints there.

Please also ask for a change to the title of your post; that will help others identify your book for you more easily.

Was it part of an anthology of short stories? If so, can you remember any of the others? When did you read it?