My nomination for the best series of private-eye novels that's now almost entirely forgotten is Bart Spicer's Carney Wilde series (I'll bet Art Scott will back me up on this). Wilde is a Philadelphia p.i. and the series covers his career from his small beginnings to his days as the owner of an agency with a number of operatives (12, in The Taming of Carney Wilde). In all the books the plotting is excellent and the writing is, too.
A few days ago I found myself sitting in a chair with three cats sleeping on me. I didn't have a book to read, but I could reach the bookshelf nearby. The book I grabbed to have a look at is the 6th in the 7-book Carney Wilde series. I'd read it years ago, and when I started it again, I knew I'd finish it. It was too good to put back on the shelf otherwise.
It opens with a shootout between a bank robber and the cops. And Wilde. A cop is killed, Wilde is wounded (bone shattered in his shoulder), and the robber gets away. There's a clue that indicates the robber might be escaping on a steamboat on an excursion to New Orleans, so Wilde goes aboard in hope of redeeming himself and not losing his agency's biggest client. Wilde is handicapped throughout because his shoulder hasn't healed and his left arm is immobilized.
There are plenty of colorful characters and twists in the tale, and Wilde meets a beautiful photographer who's destined to become his wife in the next book. I thoroughly enjoyed reading the story again, even though I remembered some of the key points. All the books in this series are highly recommended.