The Promised Land is one of the earliest Spenser novels. It's set and written in the 1970s, and the bad guys wear leisure suits. Susan Silverman is working as a high school guidance counselor. Much more clearly than in later novels, Hawk is bad: working as an enforcer, a creature from the dark side. He does some things which a good person would not do. Plus, everyone is a lot younger. They drink and eat more, they exercise harder, and they try to do the right thing. Well, okay, some of them try to do the wrong thing. But that's why they're the bad guys!
This is a novel of relationships: Spenser and Susan (just getting started in 1976), Spenser and Hawk (same), the woman Spenser is hired to find and her over-loving husband. The relationships are live, in constant action, and filled with risk. The characters think constantly. They talk about why they think the way they do. They explore each others' lives and try to come to terms with the kind of people they all are, or were, or might become. The wise-ass repartee is tremendously appealing; it's hilarious; it couldn't be better.
If you've never read one of Robert B. Parker's novels, this might be the best one to start with. If you've read them all, this is a great one to read again.