I took a look at your site and it appears you have a class assigned to the body on the Home page, but not on the Books page. With this body class you have some different text styles that are being inherited by those nav links.
Good question! In this case I took a look at the nav bar on the home page and took note of the font styles. Specifically, I noticed the Roboto font family being used was coming from the body class:
I then went to the Book page and checked the navbar there and it had a different default font family (Arial).
As CSS styles cascade down, I assumed there was a style/class applied to the body on one page but not the other. I checked the Book page Body and noticed there was no class name applied.