There are so many! I enjoy reading series as well, because of the character development and ability to "know" the protagonist for years.
Goodreads has an extensive list of individual titles in crime fiction that are part of a series.
The Adam Dalgliesh series by PD James would keep you busy for a while, if you haven't read them already. There are 14, according to wiki.
- Cover Her Face (1962)
- A Mind to Murder (1963)
- Unnatural Causes (1967)
- Shroud for a Nightingale (1971)
- The Black Tower (1975)
- Death of an Expert Witness (1977)
- A Taste for Death (1986)
- Devices and Desires (1989)
- Original Sin (1994)
- A Certain Justice (1997)
- Death in Holy Orders (2001)
- The Murder Room (2003)
- The Lighthouse (2005)
- The Private Patient (2008)
For a grittier feel, I also really like the John Rain series by Barry Eisler. The lead is an assass for hire, but somehow you root for him anyway. The way he lives his life in hiding, and the emotional toll that takes, is well written.
- A Clean Kill in Tokyo, previous published as Rain Fall (2002)
- A Lonely Resurrection, previously published as Hard Rain in the US (2003)
- Winner Take All, previously published as Rain Storm in the US (2004)
- Redemption Games, previously published as Killing Rain in the US
- Extremis, previously published as The Last Assassin (2006)
- The Killer Ascendant, previously published as Requiem for an Assassin (2007)
- The Detachment (2011)(w/ Ben Treven)
- Graveyard of Memories (2014)
If you like more of a police procedural, I highly recommend the Harry Bosch series by Michael Connelly. He also write the Mickey Haller books.