Two points. 1.) A mockup is a living document that acts as a guide to a book layout. Like I said, a guide. The size of the book will determine how type is read and viewed. Most artists and teachers nowadays take size for granted, but it is the single-most important factor in determining how a work is perceived. Imagine changing the size of a playing card... imagine what would happen if they didn't fit your hand the way they do, or if you shrunk them and it was nearly impossible for old people to see the symbols... well, you get it then. You can only learn about size through trial and error, your own eyes, and your audience. You can work with the first two easily enough. When you get to an audience, then you have to do some real decision-making.
Point two. A type story, is the fonts you're using. The way they interact makes a story. You can use several on a cover if you like. Do not use more than two in body copy, or the inside of a book or long text. You're making weather here. If you want it to snow, make it snow, don't switch to rain every five minutes. (I mean if that's what you're doing go for it, so long as you do it in an amazing way! Otherwise, stick to two fonts, because that's proven the most successful.)
2.) Do not lose track of your fonts. Once you establish the fonts, the atmosphere, mood and setting of the book's story is established. Don't change what you have. Stick with two fonts if you can. Fonts are the easiest way to distract away from the artwork, so don't let them overshadow what it is important. Unless the typography is the artwork, keep it consistent. Type acts as a kind of temperature. It is prevalent throughout the entire room, but the minute you upset it, everyone is going to notice. Are you going for a medieval Icelandic temperature riding on the edge of a fairy-tale? Then don't throw in a modern sans-serif font in the middle. If you're going for ultra Scandinavian modern, don't throw in some kind of scrawled handwritten font. You get the idea.
Keep track of your fonts. Learn their subtleties and intricacies. Get to know them. Why do they do what they do? Why do they make you feel that way? It could be that they are the same thing eyes of the 9th century looked upon with 9th century tastes and ideas. See, the past never dies; it just gets dispersed across the globe like some kind of candle exploded and caught fire all over the place. Type is like that, all over your book.
Art is about decision-making, that's all it is. Informed decision-making. You could send a trend around the entire world given enough time and thought. Half of your thought it unconscious, give it time to work. Never sit too long on any one thing. Equanimity. Disperse your attention across the desk, and the room, and even out the window like two ravens, but do it continuously with flow and effort as though you were lining up dominoes or corn kernels in a piston. When it's time, do what it is you were meant to do. Consistency is the key to any wide movement and change.