I need to make a book cake. It will be buttercream except I will have two page-sized pieces of fondant layed out that will be the open pages on the book. There will be decorations on the fondant however, I don't want to make the decorations in fondant, I want to pipe them with buttercream. Based on the simple design, I think it will look fine in buttercream. My question is: will piping the buttercream on top of the fondant ruin the fondant in any way? Technically, the fondant will have buttercream under it and on top of it. Will that ruin it? There won't be too much buttercream on top, just a few piping designs.

you'll be fine -- I've actually done the exact same scenario with a book cake -- no worries