Hey Christine!
I noticed your review for Tending Brigid’s Flame and wanted to pop in with some more recommendations! That particular book has meh reviews from Irish people but I am sorry you didn’t connect with it. The two books I can definitely recommend for Brighid are Pagan Portals: Brigid by Morgan Daimler and Brigid by Courtney Weber.
As for other stories, there is loads of information in Her lore and mythos! I’ll leave some links here for you if you’re interested in learning more. And you’re right, the ancient Irish didn’t have a writing system outside of Ogham, and that was just used for carvings on stones. What we do know about them comes from the Roman missionaries and monks that came to Ireland and wrote everything down.
The Life of St. Brigid → Bethu Brigte (it is impossible to separate Brigid the Goddess from St. Brigid, so learning about both aspects is a good idea!)
The Second Battle of Mag Tuired → The Second Battle of Mag Tuired (this one details an important battle in Irish mythology; it also tells the story of how Brighid invented Keening)
The Well at the Edge of Loch Lawne → Srath an Urláir | The Schools’ Collection | dúchas.ie (folklore directly from Ireland)
The Mantle of St. Brigid → Legendary Fictions of the Irish Celts: St Brigid's Cloak
I also have a video that talks about some of Her myths and stories. And I even wrote a post about Herself here → St. Brigid 
Things You (Probably) Didn’t Know