I don't know that this is the last of the Chee-Leaphorn books, and honestly, I hope it isn't. For one thing, I think Chee and Leaphorn need a grand philosophical send-off, and this book is more along the lines of 'happy endings for everybody'. Warning: spoilers follow, but if you've been reading the series, you know in your heart of hearts what's there already.
Along the way, it's a little sad to report Hillerman just sort of breezes through the motions. I don't think if this were a first MS it would've had a second reading from an editor. The "mystery" is easily solved, and isn't anything particularly interesting. In order to further his schtick along, Hillerman has to change the jobs of not one but two of his characters, sending Bernie Manuelito down to the US-Mexican border as part of the US Border Patrol and Cowboy Dashee (for reasons never explained) is no longer a deputy sherriff but an enforcement agent for the Bureau of Land Management.
There's all sorts of potential in the basic outline, but it's fleshed out to the approximate extent that a screen treatment might be. As I was zipping through this, for some reason I envisioned the best version of this work as a movie directed by Steven Soderbergh, along the lines of The Limey. A film director, for instance, might eliminate all the living room set pieces - Leaphorn leaves his couch and coffee pot only long enough to rummage in the garage.
Perhaps more problematic is the introduction of a cartoon villain, all the way from the cesspool of corruption, Washington, DC, who gets what we used to call 'Hawaiian Capital Punishment'. (The origin of this phrase: Hawaii 5-0. Back in the days of the TV series, Hawaii had no capital punishment, and a very large number of the episodes ended in a murderer being brought to justice by being shot by Steve McGarrett or one of his minions -- justice done in the old west style without the messy issue of the judicial system coming into play.)
The action doesn't have much at all to do with the four corners area, traditional native american culture, or the tension between the idea of a 'reservation' of traditional language and culture and the modern world which underlies so much of the best parts of the Chee-Leaphorn series.
So, the happy ending, of course, is: Leaphorn is apparently content with his relationship with Louisa; Chee and Bernie finally get together, and we know they're going to settle in the exact place where Chee's had his trailer all these years, content in traditionalism, but with a real house instead. Somewhat disappointingly, despite Hillerman's attempts to set up Bernie as capable and independent, the deus ex machina which brings them together is Chee and Cowboy Dashee coming to Bernie's timely rescue and Bernie forsaking a future law enforcement career.
In terms of sketching out the future obituaries of the characters, this at least seemingly settles the main characters' conflicts in a way which is settling to their souls. Leaphorn and Chee, representing two ends of the spectrum in terms of their beliefs in the old ways of their culture, are seemingly reconciled, or their differences are buried.
If I were writing the next book in the series, here's how I'd go about it. Hillerman should take as much time as he needs to flesh out his finale, if ever there will be one, from Sinister Pig's few hundred scant pages to whatever the text will bear. Chee needs some sort of final, ultimate crisis to reconcile his belief in traditionalism and his need to be part of the hierarchical apparatus of law enforcement; something focussed on the essential aspects of justice and what it means to be part of a culture. It should put him up in vital opposition to Leaphorn, and maybe the author can even kill off the Legendary Lieutenant to further things along. Chee could even be solving Leaphorn's murder, using his notes and effects to do so, to provide a nice retrospective of Leaphorn's career. But it's got to be something that gets to the heart of where the Navajo people are in the early 21st century: that looks forward to where they might be, surrounded by the Belagana, in a hundred or a thousand years. These are the sorts of things we look to our elders to prophesy, and what I'm hoping Mr. Hillerman will produce as a coda to this wonderful series.