I originally wrote three books that were in no way connected. During my publishing journey, I pitched "Philia" (before it was named "Philia") to an executive editor at a New York publishing house. She requested to see the full manuscript and after reviewing it called me to say she liked it. Yes, I freaked out internally. Every writer dreams of an editor calling to say she likes your book and wants to buy it. It only takes one person, folks. She asked if I'd consider turning this book into a series. How ironic that I happened to have two other books, although as I mentioned, they weren't connected in any capacity.
But, I connected them. I created a series, The Sisters of Gamma Omicron Delta, and made each of the lead heroines sorority sisters. Sororities are awesome, BTW.
I had what I'll call "working titles" on my books. A book title is as important to marketing as the book itself, and the cover. But, writing book titles, synopses, and elevator pitches are harder than writing novels, at least for me.
While I was working through the details of how I'd connect each of the books, I came to the conclusion that each of my characters represented and exhibited characteristics of a certain type of love.
Thus, the marketing titles of my books were born: Philia, Agape, Eros, and Storge. Some irony that these words are Greek for the types of love, and what connects each book to each other is that the characters were all in the same Greek organization: Gamma Omicron Delta. Love when things work out like that.