The king is a strong piece
This is the italian title of a very famous book, king power in chess , written by Edmar Mednis. I think that this book is very interesting, I usually prefer books for specific topics, and this book goes in this way. Talk about the king in all parts of the game and talk about his power, I think that is very hard use well the king and this book can help you.
In general we know that in the opening and also in the middlegame is important the safe of the king, the castle can help us.
In the endgame we must use the king , and it has an active role
This is a general rule, but in chess there are exceptions, Mednis talks about also this. I think that these ideas helped me in my games.