Okay, I gave you one credential. I served with E Recon 1/7th Cavalry. Crumley's a Nam vet too and in "One to Count Cadence" wrote one of the earliest novels that began to address that experience directly.
Here in "Mexican Tree Duck" he creates the emotional landscape that is shared by many of us who "also served."
Detective fiction has a subgenre that I might call "My Best Friend did it." Here that genre is mined to create the sense of abandonment and betrayal that many who served in Vietnam are ultimately heir to.
My favorite scene is that of Serita's rescue.
All the now old farts get it together and do exactly the tacticaly correct thing to do for what turns out, (for them), to be the inevitably wrong reason. Worse they fail to protect their unprotected flank and CW loses his love.
Doing what your supposed to and getting screwed for it....
Well every Nam vet I know understands that.
On top of that, (the not quite Masters in English is now speaking), what a technically proficient read!
Crumley knows how to use this genre and spin his good tale. He has slipped in other books, but this was not one of them, and I will still read them every one.