For much of the series, the characters in Spenser books with the notable exception of Rachel Wallace are heterosexual. Of late, though, Parker has introduced detective Farrell, a gay, and in this installment, he examines attitudes toward the gay life style. One problem, though, and this is talked out in the book, is the fact that the majority of people Spenser meets are shady in one way or another, be they of a different ethnic background, or sexual preferance, or whatever.
We do learn of an incident in Hawk's background along with a little more information as we meet his mentor and the mentor's son.
Spenser actually is working two cases here, both pro bono, one for Hawk and the other for Susan. There's irony here. In a previous book, Spenser tried to help her ex-husband, and now for one of her friends. Both times, Susan finds herself betrayed by those she thought she knew.
By the way, I notice more and more criticism lately of Susan Silverman's presence in the books. But she is an essential character. Spenser has a code of ethics and there are times that he feels he has to violate that code in order to do the right thing. This causes enough turmoil that, let's face it, the guy needs a shrink, but is very unlikely to seek one out. His falling in love with one neatly solves the problem. Hence, Susan.
So the story has some failings, but basically should give you four or five enjoyable hours while you read it.