I read this novel in the late 80’s. I do not recall the cover at all nor any part of the title, but it was paperback. I believe the author was writing completely out of his normal genre. I thought it was maybe James Patterson or John Grisham, but my own search didn’t reveal anything that felt right.
The novel follows a young prince, maybe 12 to 14 years old, who is framed for the death of the king by the king’s trusted friend, I’ll refer to him as the advisor, as in this TV Trope.
The prince was accustomed to bringing the king his meal, and the advisor poisons the meal. I want to say the poison makes you breathe fire causing death by burning the victim’s body.
The prince was playing in a secret passage and I believe looks through the eyes of a wall painting and witnesses the advisor poison the king and making it look like the prince did it. The prince is arrested, despite protest of innocence, and thrown in the dungeon while his very young brother is made king.
The new king is mostly the puppet of the advisor and does things that make him a much unloved king. Somehow the prince escapes the dungeons, though it takes many years. The prince then confronts and defeats the evil advisor and is made king. The younger brother who was king does not have a happy ending, but I believe lives.
Additionally I recall a part of the novel, maybe a chapter, from the perspective of the family dog.