I'll nominate
A Catskill Eagle by Robert B. Parker. It's not the first in his Spenser series, but it is one of the best (you can read this series in any order, though the developing relationships are fun to follow chronologically). The friendship I am nominating this for is that between Spenser and Hawk, an African-American solider of fortune and unquestionably the greatest badass ever created in crime novels (IMHO).
In this story a letter tells Spenser that Susan (his long time partner) is in trouble and Hawk is in jail, and from there it’s a foregone conclusion on all hell breaking loose. The bonds between the three are never stronger, the story has never been more intense and the action has never been so defined. It’s a masterfully written book that could easily stand alone, not as minimalist as later Spensers or as hard-boiled as earlier ones, transitional both in terms of his style and the way the characters develop.
Amazon US $7.99
Amazon UK £4.99
Amazon AUS $11.99