What is Spenser's connection to Martin Quirk in the novel, The Widening Gyre?

Quirk is Spenser's contact within the Boston police department. A busy, well-regarded detective, Quirk has a smart mouth, especially when dealing with Spenser. However, Quirk is also intensely loyal and does not mind doing Spenser a favor or two outside the law so long as he knows that Spenser's intentions are true.