I recommend any books in the Hardman series, but start with the first one. As you can see from the cover, they were marketed like any other men's adventure series from the '70s, but they were a cut above. Sure, Dennis wrote fast, but the writing's very good for the most part. Some books are better than others, but they're all fun if you like hardboiled p.i. stories. The setting (Atlanta) is different, and Hardman himself is an intriguing character. He has a black partner, Hump Evans, who's a bit like Parker's Hawk, and I wonder if Parker read this series.
From the back cover: [Hardman's] laws of right and wrong are his own, and he's willing to live and risk dying by them." The case involves "a beautiful blonde, a Black Mafia leader, a white politician, and a murder who strikes at victims regardless of race, creed or sex."
If you haven't read these books, you've missed a treat.