This series is by a British author. I have only read the first two books, as those are the only ones available in the USA at this time, so I can't speak regarding the appropriateness of the remaining books in the series.
With that said, I LOVED the first two books! The first book is Lockdown. Alex is admittedly a bad person. He has his peers at school scared he will take their lunch money. He breaks into houses for fun. However, he is framed for a murder that he did not commit and sent to Furnace, a solid rock prison a mile beneath the earth's surface. The inmates have no hope, they will all serve life sentences. Prisoners are occasionally taken from their cells and used for science experiments that seem to go horribly wrong. However, Alex and his new found friends see a glimmer of hope and attempt to make a break from the prison.
The second book is Solitary. This one gives the reader a deeper understanding of what goes on in the laboratory at Furnace. Someone is up to some very evil science experiments and the outside world has no clue of what is going on down there. Furnace is not a state or federally ran prison, it is independently owned and operated. This book kept me on the edge of my seat and I can't wait until the third one comes out in August!
I would classify these as science fiction thrillers. I know other places have also classified these as horror, but I'm not familiar enough with that genre to make that assertion. They do have some violence and gore in them. At this point, I would say they are appropriate for middle school readers through adult readers. However, due to the fact that I work at a Christian school and I've read the Amazon UK reviews for the next three titles in the series, I feel that I must hold off putting them in my classroom library until I've had a chance to read the remaining books in the series. I'd hate to get the students excited about a new series only to find out later that it ultimately was not appropriate for middle school students.