In the previous two books in this series, a lot of time was spent bringing players into place and setting up introductions. There is plenty of action, but it all retains a fairly light touch - there is a sense that, although things are happening and changes are occurring, nothing really major has changed yet, and anything that has changed is not necessarily permanent. This is common in series, when you start to look at them holistically; it takes time and words to put all of the characters into the places they need to be for the plot to start really picking up. With The Stiehl Assassin, the plot definitely accelerates.
There's something vaguely amusing about the fact that as I go through and read The Fall of Shannara series, I find myself most interested by, and rooting for, the invading Skaar and their hyper-competent Princess Ajin. It's not that I don't like the other characters (most of them, anyway), or that Brooks hasn't done a good job at making them sympathetic. To me, though, Princess Ajin is exactly what the best characters should be.
Where do I even start with a Shannara review? Shannara is epic fantasy in the very literal sense of the word, spanning hundreds of years of in-world history across myriad series and trilogies and stand-alone novels. Perhaps Brandon Sanderson's Cosmere may eventually be larger in literary scope, but even that will likely not sprawl so much as Shannara. Where a series like Wheel of Time covers a single story arc, Shannara has era, ages, and dozens of independent arcs. Sometimes, one has to wonder if Terry Brooks can bring himself to write anything that isn't Shannara: supposedly his Knight of the Word trilogy began as something new, and morphed into a prelude to Shannara.