Honey, I shrunk the mission briefing book!

You can minimize and maximize the size of the mission briefing book on your map screen by double-clicking on the briefing book's outer right-hand edge.

Minimizing it has the benefit of letting you see more of the map easily. However, the text in the briefing book becomes illegible when minimized. That's why I personally prefer to simply mouse-drag the briefing book over to the map screen's extreme left when I need to view map details behind it.