When decorating, if you want to make the biggest impact, it's smart to keep collections together. Not only does it look good, it helps keep things organized. Even though books aren't technically a collection, there was no reason for me to have them spread out on two shelves when I could fit them all on my big new shelf.
Grouping a collection together makes a stronger style statement and is risk free (if you hate it, move it back!) and looks great!