The Diamond of Darkhold by Jeanne DuPrau
In the village of Sparks lived the remnant of the people of "Ember". In the time since they left their home in the underground city and traveled to this new place many hardships have fallen on all of them.
When a traveling sales person sells a partly damaged book with missing pages Doon, trades for the book. Doon is surprised to learn that the book is from the City of Ember, left by the founding fathers for the coming generations. The book, hard to understand because of the missing pages presents a mystery that Doon sets out to solve.
Eight pages are what is left for Doon to try and decipher. Doon convinces Lina that in order to get more information they must return to the City of Ember to get the answers they need. So it was decided that would travel again the same road back to Ember, where they hoped to find not only what was meant for them but also thing that were left behind that could now ease the hardships of all those who lived in Sparks.
An so they set off on a mission that will bring them to the place they love most "home" . Will they make it, to return or will something go terribly wrong...
You must choose to read the rest but will find this a rewarding read as and end to the series. To start at the beginning read: Book 1..."The City of Ember", Book 2..."The People of Sparks", Book 3...The Prophet of Yonwood".