The Diamond of Darkhold (the Fourth Book of Ember) by JeanneDuPrau
"The Diamond of Darkhold" brings us to the end of the tale of Lina and Doon and the Emberites. You may have noticed that throughout my review of "The People of Sparks," I never used the term "People of Sparks," nor, to my knowledge, did I use the term "People of Ember." This is because at the end of "The People of Sparks," the Sparksites decide that the Emberites are also "People of Sparks."
"The Diamond of Darkhold" opens at the end of the Emberites' first winter in Sparks. It has been a very hard winter, made worse by the destruction of some of their food stores when the roof of the Ark collapsed. The Emberites have made a collection of wood and windows and doors scavenged from other buildings so that they can use them in building new houses to live in. While working on the houses, Loris, Doon's father, trips over something and slashes his hand on a window, getting glass embedded in his hand in the process.
The removal of the glass from Loris's hand is distressing for the characters, to say the least. By the time they make it to Dr. Hester's house, it is nighttime and Dr. Hester has gone to make a house call, so first Mrs. Murdo and then Dr. Hester, once she arrives, have to work by candlelight. To make things worse, Doon, Lina, Mrs. Murdo, and Loris's first thunderstorm hits while they are working, and as a result, everyone jumps every time the lighting flashes.
The next day, a new roamer begins to sell her items. She has a few bedraggled sheep and assorted other random, mostly junky, items, and she tells them that she will only take food for the things she is selling. She does have three useful things, though. The first two are pieces of news -- that there is a pack of wolves in the area, and that a new star has been seen in the sky. The star is greenish in color and moves much more quickly than stars should move. The third thing she has are the last eight pages of a book that has "For the People from Ember" written on the cover. Lina gets the roamer to accept a match in trade for the book, and Doon takes the book home to read.
After reading the pages, which are highly technical and don't make much sense to him, Doon convinces Lina that there is something important that the Builders left behind for them and that they should return to Ember to look for it. Doon and Lina reason that, even if they cannot find whatever was left for them, there are probably things they could bring back, such as food, or antibacterial ointment for Loris's hand, that would help the people of Sparks.
So they make the return journey. Since they are young and healthy, they can make the return journey in only one day, as compared to the three it took them when they left accompanied by hundreds of other people, some of whom were ill or elderly or both. They spend the night in the cave above Ember and the next day, using the generator that Doon built at the end of "The People of Sparks" for light, they head for the city. They make several discoveries this day. One is that there is a pathway from the cave to the Unknown Regions outside of Ember. The other is that someone has found Ember and is living there.
Doon is taken hostage by the Troggs, the family who is living in Ember, and Lina is on her own to get back to Sparks to bring someone to help rescue Doon.
The book ends with an overview of what will happen to Sparks, and to Lina and Doon in particular, in the years following this book. There is even one flash forward several generations to what will be on the site of "the city" that Lina visited with Caspar and Maddy in "The People of Sparks." This is overall a very uplifting book. Although the characters admit that "happily ever after" is not really a possibility for humans, happiness is, and they have found it in Sparks.