I like a good series character in my crime fiction, and no one has offered us more of these than Block, and they cover a range of styles.
Matthew Scudder (16 novels) is a private eye in NYC, whose best friend is a hardened criminal. The novels vary in tone and story, some tough to the point of brutality, but morality is always complex, and Scudder being a recovering alcoholic plays a big part. These are worth reading in order, mostly, because the character does develop (including getting married, eventually).
Bernie Rhodenbarr (10 novels) is a professional burglar who also runs a bookshop. There is a formula here: Bernie commits a burglary in his usual skilled and careful manner, but finds himself prime suspect in a murder. The rest of the novel sees him evading the police while trying to solve the crime, ending with a traditional “I expect you’re wondering why I’ve called you here today” scene. The pattern means I wouldn’t recommend reading a bunch close together, but they are always highly entertaining.
Evan Tanner (8 novels) is an outrageous character, a kind of freelance international spy and adventurer. He never sleeps (shrapnel in the brain from Korea) and uses his time to write doctoral theses on any subject commissioned, and to support any and every bonkers lost-cause society. An interesting twist was when Block revived Tanner (from suspended animation!) 20 years later, to find that many of the apparently hopeless European independence movements had finally won through. These books are wildly over the top in what Tanner achieves, and may not be to everyone’s tastes.
Chip Harrison (4 novels): these start as coming-of-age novels, then Chip becomes the assistant of an eccentric but brilliant detective who never leaves his home (an homage to Nero Wolfe). These are the only ones that absolutely need to be read in order, as a tetralogy.
Keller (4 books): these are less novels than collections of episodes in the life of this professional hitman, often given complicated jobs, more often complicating them himself by identifying with his targets and their lives.
There are others: I particularly like Martin Ehrengraf (a bunch of short stories), a lawyer with a perfect record of getting off those accused of murder. It’s never stated outright, but it’s unmistakeable that his main technique is committing more murders with an identical M.O. or to frame someone else – the most casually immoral protagonist I’ve ever seen in a series.
There are of course also non-series books and stories. I’ve never read a poor Block (I’ve read over 50), perhaps because he is a consummate craftsman, whether being hardboiled or funny. He’s written I believe four books on writing. I recommend him very highly – I’d suggest starting with a Scudder, unless one of those other descriptions especially appeal.