By the 9th book of the Wagon's West series, the formula has been pretty established. Characters will go to a new area, there will be a bad guy there, and one of good guys will get the better of him. There will also be a new love story as a character that hasn't previously gotten married will end up getting married. And a few new characters will be introduced.
Washington! feels more important than the other books I've read in the series outside of the first one. It features the end of two major characters storylines, people who have been around a long time. It also brings three of the main characters into an interesting love triangle that is a bit reminiscent of Whip, Eulalia, and Cathy, but instead of being totally familiar, those characters actually share their experiences with the younger generation and when it plays out you're not sure things will work out as they have before.
The bad guy is a pretty standard one for these books, but the final confrontation between him and Toby is pretty well executed. The new characters that are introduced, Bettina and Clarissa are fine, and I'm guessing Clarissa will be a major character going forward. This book sets up Montana pretty directly, and it's clear that the story of the Holts, Blakes, and Brentwoods will be continuing through that one pretty directly.