The Sanctuary is a story about love. It is not a romance, but love is the foremost ideal in this sometimes brutal tale, which mostly takes place inside a prison (indeed, this novel sometimes feels like a treatise on the prison system in America, but the message of love still shines beyond that).
The Sanctuary completes the story and character arcs that began in The Priest's Graveyard. I would definitely suggest reading the books in order. Doing so isn't necessary to understand the story, but you cannot properly appreciate The Sanctuary without knowing where the characters Danny and Renee are coming from.
If you've already read The Priest's Graveyard (even if you didn't like it), reading this sequel will likely satisfy any disappointment you may have felt at the ending of that book.
If what was described as punishments is real and is happening in our prison system. It would be a very bad place to be. As far as recommending this book to a friend, I would have to really think about it, the contents are something everyone can handle. It might have come with a warning about the violence portrayed.
The Sanctuary was a dark story. No doubt about that. In fact, I needed an occasional break from it; it was that dark. However, in typical Dekker style, redemptive themes made the payoff worth it. I really enjoyed The Priest's Graveyard, so this was a great sequel. There are twists in it that I didn't see coming. When it comes to the depravity of man, Dekker is a master at showing it. But he also shows how love is greater than all of the darkness. I love that about his stories. Anyway, I enjoyed this one. I don't want to tell too much and spoil it for readers, but let me say if you enjoyed the first book, you'll find this one worth your time as well.