1) Ariadne and the Beast: A Short StoryA little more than 25 years before Helen of Sparta. I guesstimate Theseus is in his late teens during the events of this story!2) Tamer of Horses~20 years before Helen of Sparta. Theseus and Pirithous are both kings of their respective domains, but still learning what that means.3) Helen of SpartaTheseus and Pirithous are both in their 40s and Heracles is older, still.4) By Helen's HandPicks up right where Helen of Sparta leaves off. Pirithous's son by Hippodamia, Polypoetes, is of age to take over for his father as king of the Lapiths.4a) A Broken HorseThis novel is written but not yet published--I'm looking for an agent!--and follows both Paris and Polypoetes as they navigate the Trojan War.5) The Siren's Song/A Sea of SorrowTakes place after the close of the Trojan War, as a retelling of The Odyssey. A Sea of Sorrow is in continuity with A Song of War, History 360's retelling of The Iliad, but my revised author's cut edition of The Siren's Song novella does exist within the same world as Helen of Sparta.6) Son of Zeus~3000 years or so after Helen of Sparta, this novel takes place in our modern world, and Pirithous is still in his 40s after being trapped in the chair of forgetfulness for millennia.
All these titles, with the exception of Helen of Sparta and By Helen's Hand, were written to stand alone. You don't need to have read Tamer of Horses to enjoy Son of Zeus (I wrote Son of Zeus immediately after I finished writing Helen of Sparta, actually, before I had even written Tamer of Horses or By Helen's Hand.) And you don't need to read Ariadne to enjoy Helen of Sparta, though Theseus appears in both.
You probably DO want to have read Helen of Sparta before you pick up By Helen's Hand, though, as they were written as a duology of Helen's life leading up to the Trojan War. And if you did happen to read Tamer of Horses before or after Helen of Sparta, it might enrich the experience of both books! If I've done my job right, you shouldn't need to have read either of Helen's books before A Broken Horse, either, when/if that makes it out into the wider world. Do I think you'll enjoy it more if you have? Yeah, of course. And even more again if you've read Tamer of Horses as well, since that's the story of Polypoetes's parents. But it shouldn't be a necessity.
Consider them all just--companion novels, that allow you to travel a little deeper into the world and the stories of the characters you've met, either in reading Helen of Sparta, or in your own exploration of the myths themselves.
Tamer of Horses, Son of Zeus, and Ariadne and the Beast are all available in wide release through the digital retailer of your choice. Tamer of Horses and Son of Zeus are also both available in paperback for your physical reading pleasure--you should be able to order them from pretty much anywhere. (Here's a bookshop.org link to prove it! Alas, the Son of Zeus hardcover is Amazon Exclusive.)
Ah! and before you go, The Siren's Song is going wide this week! Look for it in your library apps and available from non-amazon vendors at this link very soon!!!