On the Edge of the Knife. Xs and Os (set)

"Xs and Os" is a fantastic series of books in the dystopian genre by British writer Malorie Blackman.
"Xs and Os" is the first book in the series, while "On the Edge of the Knife" is the second.
In the first book,... we are told about an alternative world where society is divided into two classes: black Xs and white Os.
Seffi belongs to the upper class of black Xs, while Callum is a white boy from the lower caste of Os. Seffi and Callum have been in love since childhood, but their union is forbidden. Maintaining their relationship becomes even more difficult when Callum's connection to the Liberation Militia, which is trying to overthrow the ruling elite, is revealed. After all, Seffi's father is a high-ranking official.
In the second book, Seffi gives birth to a child by Callum, but her child is a half-breed and is in constant danger. To protect her daughter, Seffi is willing to go to great lengths — even to provide an alibi for Callum's brother, Jude, who has committed a crime.