While it may appear that you cannot change fonts or fonts sizes in Apertures book layouts, you can. You can select any font or font size you want for any text in an Aperture book. Heres how:
Simply click COMMAND T to show the fonts dialogue box. In this box you can change the fonts size, color, etc.
You can go to the size box and type in any size you want or you can use the provided font sizes. Be sure to use only whole numbers. In this box, you can also change the vertical alignment of the highlighted text, and the button in the upper left (T) creates a drop down shadow.