The Lost Baron(book).

This is another book written by Allan French.
It's another great mystery book, and a good read.
Here is a preview.

 Martin, son of Sir Anselm of the Hollow, risks his life more than one time, in this fast-paced mystery of 1200. The good king Richard is dead, and his brother John is king, and will do anything to get money. Suddenly, Baron Eric disappears, and his moody and jealous cousin Basil moves in. His daughter Rosamund and wife can do nothing to stop him, so the castle is in danger. In an unguarded moment of kindness, Basil invites Martin to become a page in the castle. Will Martin take the chance and somehow save the castle and Rosamund? Or will he fall into the hands of a dreadful foe?

It's a really good book.