My brother strongly recommended Herron to me. I read and enjoyed Slow Horses, mean to continue with the series, but keep forgetting. Once read a Herron non-Slough House novel (Down Cemetery Road), which was OK but less good than S. H.
I liked Joseph Heywood's Woods Cop series, set in Michigan's Upper Peninsula. Series ended in 2011, don't know if Heywood is still writing.
Bill James's (Welshman, not the sabermetric guy) lengthy Harpur and Iles series is excellent, but it's best to start at the beginning and I fear those books are hard to find.
Two other long-running series I've always enjoyed are Lawrence Block's Matt Scudder books (NYC) and Loren D. Estleman's Amos Walker series (Detroit).