If assassins count as hitmen, there is also the Destroyerseries by Murphy and Sapir. The early ones can be charitably described as “not entirely unreadable.”
There is also The Day of the Jackal, which I recall as a very good thriller.
I am talking about books series where the hitman kills people simply because it is a well paying occupation, not because he kills people because of revenge, because he enjoys killing people, because he is righting the wrongs of the world or because he is a government employee furthering government policy.
Then Keller is exactly what you want. It’s just a job to him, and I can’t recall an instance where he felt all that strongly about it, even when it was for revenge (Hit and Run). The original short stories were Block playing with the idea that your typical guy could be a killer - the guy at your gym, out walking his dog, or having a drink at the local bar. Being a hitman is just what he does, not who he is. Who he is, for the most part, is a stamp collector. 
There is also Pest Control and The Exterminators by Bill Fitzhugh:
For another funny take on hitmen-for-hire. 
Though in this case - inadvertently.
You might try Mark Dawson’s John Milton series. He’s a self-published author who’s a huge success story (bestselling author, six-figure income from his books).
http://markjdawson.com/ if you want to join his mailing list for some free books, or else he’s on Amazon.
I have read all of Keller, and more than three quarters of Butcher’ Boy, and imo keller is MUCH better.
Keller is the study of a quirky personality, a dreamer who happens to kill people while he stumbles through life.
TBB is a procedural, all about the nuts and bolts of killers and investigations. Its very good, but it seems like something I’ve already read a thousand times m
For something a little different, there are the Miss Melville novels by Evelyn Smith. I haven’t read them myself, but a friend thought they were amazing.
I’ve bought the second Keller book. I’m not going to buy any more of The Butcher’s Boy books.
Damn, I’m gonna have to look into Keller, because Butcher’s Boy and Parker have long been among my all-time faves!
Parker’s personality is closer to Butcher’s Boy, but in my mind the books are more about Parker and his ethos. Which also puts the books closer in feel to the Keller one I’ve read, even though their personalities are about 180 degrees opposite each other.
The amusing thing about Keller is that the author is able to make the guy completely sympathetic. As a reader, you catch yourself rooting for the fellow - only to remember with a start that he’s killing people for money. 
I’ll second that Parker is not a hitman. He is a heist-man. Invariably, most of his heists go wrong, usually due to members of the team double-crossing him. The magic of Stark’s (Westlake’s) books is the planning, execution, and then improvisation by Parker to get away.
Keller’s stories include the similar planning and execution (no pun intended) of his assignments, sprinkled with the mundane life of a contract killer that loves to collect stamps.
James Mitchell’s Callannovels, also the basis for a TV series. The premise is that Callan is an agent for British Intelligence, he uses tactics such as blackmail, framing and assassination. He hates he job, but they won’t let him leave.
Oops, didn’t spot that bit. My bad.
I’ve now read through all the Keller books, and they’re really fun. I’ve enjoyed them a lot, though the last one ends with something of a cliffhanger, which none of the others do.
What I especially like is that each of his murder excursions is written as a nearly stand-alone short story, with a half-dozen or so collected in each book. There is a chronology and a narrative tying the stories together, mostly references to previous jobs, so it’s helpful to read them in order, but not necessary.
Lots of droll humor, lots of fun.
If you like these books, Block has two other series which I’d previously read and liked a lot. The Burglar series and the Evan Tanner series, both of which are a lot of fun. He is probably best known for his Matthew Scudder series about an alcoholic detective that’s more downbeat and I’m not as fond of.
Matt Helm of the Donald Hamilton novels.
The books are nothing like the movies.
Matt Helm is a government agent, not a professional hit man.
I believe that this was the basis for the “not entirely unwatchable” movie Remo Williams: the Adventure Begins and the short-lived TV series of the same name.