The Diamond of Darkhold brings the Ember tetralogy back to the two main characters' arcs. The third book, Prophet of Yonwood, was an enjoyable standalone, an extended diagonal flashback explaining obliquely (kind of a shallow trick, but an enjoyable read nonetheless) how the world of Ember came to be.
It's also on some level an elaboration of du Prau's social criticism, which is not complicated in the Books of Ember, but is appropriate (I think) for the YA reader.
Still, it departs from the storyline, and though enjoyable in its own right, left me a bit cold because I was eager to either find out a lot about what happened to our world so that it became the world in Ember, or what happened to Lina Mayfleet and Doon Harrow after the end of People of Sparks.
Diamond of Darkhold satisfies the second goal best, and does it in a style that I think may be du Prau's alone. I'm not sure what it is exactly, but the Ember books have a slow/quiet/small feel to them. It's not that there isn't danger, that characters are dull, or that the events have only local significance.
In fact, what I'm feeling may be the afterglow - the dull, orangey afterglow - of the movie City of Ember, which I saw before reading the book. I hardly ever do that, and this is partly why. I loved the movie, and the books, but I can't disambiguate them. The movie has a gloomy, washed-out feel, with dull echoes and worn edges to everything. Bill Murray is brilliant, though physically wrong for the part of mayor, and the screenplay (except for a large creature) sticks very close to the book.
The movie stayed with me when I read the first book. For Sparks, Yonwood, and most of Darkhold, I was able to generate my own visuals, but I am not sure what influence the movie may still have had.
What I mean by slow/quiet/small is really about the characters, anyway. I like Lina and Doon. They are thoughtful, considerate, quiet, and brave. They remind me of bookish kids I either studied alongside or much later taught.
They contrast starkly with a similar age pair from Pullman's Dark Materials trilogy, Lyra Belacqua and Will Parry. Lyra is all impulse and anger and bravado, while Will is emotionally shackled to his dependent mother, mentally ill and under her pubescent son's protection. Everything Lyra does is larger than life, and the world is (or rather, worlds are) crowding around Will with immeasurable menace.
Lina (not "Lynn-uh" but "Line-uh," almost rhyming with Lyra) and Doon face dangers and obstacles in an intially much smaller universe. It's almost claustrophobic. But even when they emerge, the wide world is somehow narrow (constrained by travel on foot, their Emberite imagination, the limited horizons of everyone around them?).
I'm not sure I'm putting this clearly. Possibly because I'm not seeing it clearly. I'm anxious to read a book of du Prau's outside the Ember universe to see if that feeling carries over.