I have completed a first (sloppy but complete) draft of the third book in the Persephone series! And yes, this will be the final book in the series, unless I do a spinoff someday, so it will for now wrap up as a tidy trilogy. Therefore, if you're the type of person who doesn't like embarking on reading a series that's yet unfinished--and I understand if you are, because I tend that direction myself--then it's okay, you can start mine now.
The title is likely to be...
If it were a stand-alone, I'd go with Immortal Spring, but to line up in nice parallel fashion with Persephone's Orchard and Underworld's Daughter, we're tacking on the singular possessive.
The first draft is now in the hands of my fantabulous beta readers, so they can help make it as un-dumb as possible. This is one of the fun parts. I haven't yet been over the manuscript so many times that I want to burn it. That comes later.
Meanwhile, you can add it to your shelves on Goodreads, though there isn't much to the listing there yet.