If you want an author in the same category as Sue Grafton, try Marcia Muller's P.I. Sharon Mccone series, there are 23 books in the series so far.
Nevada Barr writes a series of books about a national park ranger that is quiet good and is in the J.D. Robb category. Jonathan Kellerman writes a series about a phychologist, Alex Delaware who consults with the police to solve crimes similiar to the James Patterson's character Alex Cross.
