In the 1970s I read 2 or 3 books from a series which were old books from my parents.
The main character was a boy (maybe Peter) whose father was part of a resistance movement. The father was captured, tried as a traitor and sent to jail.
The boy (who appeared to have no living mother) was taken to live with a senior person (count perhaps) who was important in the government or people the rebellion was against.
This “count” was seen by the boy as dark and evil and the grand house seemed like a prison to the boy.
The boy although very unhappy eventually made friends with the “count’s” very young daughter, went horse-riding, played etc. But he tried to get messages to the resistance movement and maybe even tried to get away. The “Count” found out, and there was some discussion about holding the father as a “hostage” to the boys good behavior or the boy to the father’s behavior.
The “Count” and the father were both really trying to work towards the same end, but the father was doing it by rebellion, and the “Count” from inside the system.
The boy ends up being treated well by the Count – but I never got to the end of the series (I think I read book 1 and 3 maybe).
I think one of the adult’s names was Sebastian and I got the feeling that it all happened in middle Europe, black forests and castles.
I would love to find out the author and the name of at least one of the books.