Hey Sterling... yes, the shelves are adjustable. However, you'll only be able to fit 4 across. So you'd be able to get 20 boxes on the unit.
If you have a Costco near you, go there. They have similar shelving, but they come with 6 shelves instead of 5. They also have a little "lip" on the back as well as wheels if you choose to put them on.
Also, the main height rods are in pairs. So, if you'd like to "stash" some books in say a closet. You could go with half the height and double the width. good for under windows (wouldn't recommend though) or anywhere else height is an issue.
Hope this helps,...
John