Each of Steve Hockensmith's four _Holmes on the Range_ books does something different. _The Crack in the Lens_ is more realistic, grittier, and much darker than any of the previous volumes. Even Big Red's trademark smartass narration gets a tad subdued in spots.
This isn't a a drawing-room mystery; it's a descent into dark places. There's a hint of Carr's _The Alienist_ here, maybe even a little _The Silence of the Lambs_. The shift adds visceral impact, but it makes the book less fresh and original than its predecessors. I must admit that I personally have had my fill of this plot; it veers toward a cliche, although setting it in the Old West is at least a shock.
Nonetheless, this is a rather ambitious book. Starting with the title, _The Crack in the Lens_ sets up a conflict between intellect and emotion, reason and faith. It doesn't entirely resolve the conflict, however. For instance, a couple of the characters make some pronouncements that really demand to be either affirmed or refuted. (They're not sympathetic characters, although--give Hockensmith credit for this--one of them turns up on the side of the good guys at a crucial point.) Even the climax is a bit ambiguous in that regard: it's as much a flash of intuition as it is logical deduction. To put it another way, this book shifts noticeably towards the real world, where things are lot messier than in the classical mystery.
It helps that, for the first time, the Amlingmeyer boys have a personal stake in the outcome. No longer are they agents of an abstract justice, or simply feeling the bite of ambition. This added dimension allows Hockensmith to sharpen up his philosophical point and display some deeper characterization. And there's still impeccable research, fast pacing, action, vivid period color, and some laugh-out-loud funny bits. If you're new to the series, though, I'd mildly suggest not starting here; _The Crack in the Lens_ will have more impact if you're already familiar with Our Heroes beforehand.
Hockensmith is taking a considerable risk here, for which I commend him. By taking this bleaker tack, he may alienate fans of his earlier books. Me, I'm not alienated--heck, I'm impressed by his chutzpah. I still plan to buy the next one the day it comes out.
Some of the charm is gone. Whether the extra realism and ambition make up for that is your call. Buy the book and find out.