Gregor and the Marks of Secret
This fourth installment in the Underland Chronicles, Gregor and the Marks of Secret written by Suzanne Collins, begins with Gregor looking at the scars left all over his body from his last adventure in the Underland. In that book, Gregor and the Curse of the Warmbloods, his mother went with him and ended up critically ill from a plague that affected the humans, rats, and other warmblooded creatures in the Underland. Gregor's mother is still recovering in the Underland.
Gregor returns to a secret area beneath Regalia on a regular basis both to have lessons from the rat, Ripred, and to choose an "artifact" from the museum to sell and pay bills. (Gregor's father is still recovering from his time Underland and can not yet work.)
Now Gregor and Queen Luxa must once more journey through the Underland on a quest. There is another prophecy, contained in a children's game rhyme, that fortells a great battle. The Bane, a white rat Gregor spared as a baby in a previous installment, is now a "Young Adult" and causing problems for many of the other creatures.
Gregor and the Marks of Secret doesn't really end. They are on the cusp of a battle to determine the fate of the Underland. The young Queen Luxa and Gregor each must fulfill a destiny, complete the prophecy, and possibly be destroyed with it. I am eagerly awaiting the next, fifth installment of the Underland Chronicles, to find out what happens to Gregor, Luxa, Boots, Ripred and all the others.