"...it’s one thing to refuse to use spells ourselves, and it’s another thing entirely to talk of deliberately bringing in a lot of grubs in order to destroy the natural magic in our settlement lands forever." [loc 1522]
Across the Great Barrier picks up almost immediately after the events of The Thirteenth Child, in which Francine 'Eff' Rothmer learnt that she wasn't cursed or wicked, and that the trouble she was having with magic wasn't a flaw, but indicative of a new methodology. In the previous novel, Eff was instrumental in dealing with an infestation of magic-draining mirrorbugs. Now, the people of Wrede's Columbia have to cope with the ecological aftermath.
The Thirteenth Child, set on a North American continent empty of human life until the arrival of 'Avrupans', was criticised for its erasure of Native Americans: but how do you -- how can Eff -- explain the absence of something that's never been there?
However, Eff is growing up (she celebrates her twentieth birthday in Across the Great Barrier) and her perspective is broader than it was in the first novel. There's more discussion of the rest of the world, and of the history of Columbia. It's interesting to examine Wrede's worldbuilding, the chains of cause and effect that produce Eff's world. Here, the 'last' Ice Age never happened, so there was no Bering land-bridge by which humans could cross to the American continent. The magical wildlife of the far West is fierce enough that dragons flee it, never mind mere humans. Lewis and Clark's expedition was utterly lost. Men who attempt to cross the Rockies come back mad, if at all. And only the Great Barrier Spell, created by Franklin and Jefferson, preserves the east of the continent from the worst of the magical perils.
Eff finishes school, works awhile in the university's menagerie, and then volunteers to head west as assistant to Professor Aldis Torgeson, a Vinlander and a biologist, who becomes something of a role model for Eff. Their expedition also includes the Aphrikan Washington Morris -- a.k.a. 'Wash' -- who acted as guide on the journey to discover the mirrorbugs.
Meanwhile Eff's twin brother Lan has returned early from boarding school, after an incident he won't discuss which has clearly shaken him to his core. His (and Eff's) friend William has been disowned by his father. Both young men join the expedition. Needless to say, they find adventure aplenty beyond the Great Barrier Spell -- not to mention traces of a mysterious threat which menaces the peaceful pioneer settlements east of the Mammoth River. The risk is compounded by the fact that some of those settlements, including one where Eff's sister and her husband live, are Rationalist: they disapprove of using magic, and the more extreme among them are even talking about deliberately reintroducing mirrorbugs to drain the natural magic from the land. Eco-crisis ahead!
And as though the ecology of magic, and the wild frontier itself, aren't enough, Eff finds herself being courted ...
One vexing profreding issue: someone had performed a number of global search/replace operations in the source for this book, leading to such infelicities as "I could see he liked being the indent of attention" [loc 3917] and "I saw a bump in the crt of the lizard’s forehead" [loc 4229].
But that didn't stop me going straight on to the final volume of the trilogy.