Due process
The idea of due process is built into the United States law in the Constitution which states that the government will follow its procedures for every person who is alleged to have been involved in a crime. This book focuses on the due process that was not allowed to Slahi. The process that was "due" to him by his human nature (according to US law) is a court proceeding to determine his guilt, where he can be represented by a lawyer.
However, since he is a prisoner of war, the process might be different, but in any case, there was a process that the US government is obligated to provide Slahi so that he knows why he is imprisoned, how or when he could be freed, and so that he cannot be punished until he is convicted of a crime, and then not in a "cruel" or "unusual" way. These processes were certainly not afforded to him.
Innocence
In America, our law states that each person is created equal with the presumption of innocence, meaning that if a person is allegedly involved in a crime, the government must assume they are innocent until they are proven guilty in a court of law.
In this book we learn about Slahi's case, when the US government failed on those fundamental convictions, deciding that Slahi's potential guilt was so important that they decided to remove the presumption of his innocence. He was already considered guilty when he was detained, so the torture he suffered ended up being more like brutal punishment for a crime he had not been convicted of.
The inhumanity of torture
For people who don't already know about the United States's controversial history of torture, the truth comes as a hard pill to swallow, but for people like Slahi, it isn't abstract, it's concrete. Much of the novel details accounts of the brutal torture, manipulation, and abuse that Slahi suffered at the hands of the US government. Not to mention that the people who were calling those shots were high government officials, so the issue of oversight is dubious.
The types of torture that the US government used at Guantanamo include severe psychological torture, humiliation, harassment of the senses, and then also bodily torture. The story explains these things (when they haven't already been redacted) to explain that the US policy was beyond unfair or cruel, it was downright evil and inhumane.
