The reason varied heights create a more organic look is because the eye doesn’t want to stop on a flat line. Three objects of the same height will make a shelf look boring even if the textures or colors are perfect. The eye has no place to rest as it scans across that line. This is usually seen with matching candlesticks, identical frames or small vases that are all of the exact same height.
Do a little experiment with one tall object, a medium object and a short object. You could use a vase as the tall item, a frame or book as the mid, and a bowl or candle holder as the short item. Group them closely so they are seen as one, but leave a small space between each object. Shift the tall item from the center to either side and the balance will change.
Why does changing height look better? Because it gives your eye somewhere to rest. The tallest object draws your attention first, and then the medium and short objects create movement down or across the shelf. You don’t need bright colors to create this visual rhythm. The tallest object doesn’t have to be your focal point either. It just needs to complement that center or anchor piece, and not compete with or block it.
Books are great when you want to give something a slight lift. If you are placing something like a small candle or a bowl on the shelf, place the book horizontally underneath the candle or bowl to give them a new level. Just make sure you match the height of the book stack with the height of the object; having a small thing sitting on a huge stack will feel weird. Same goes for placing an item like a vase that weighs more on just a single book, which will make it feel off. You could put the spines of books facing away, especially if their colors or patterns don’t match your intended color scheme. You could turn one out, or put one up that matches one of the accent colors on the shelf.
Don’t create a staircase. If each item gets taller or shorter by the same amount, the look is as structured as a line. And the same goes for spacing, which should also be varied. Let one piece overlap another from your main view of the shelf. Maybe have a piece sit slightly forward of that other item, like a small one that sits behind. And of course keep in mind to always leave some negative space around your arrangement. This will allow the edges of each piece to be visible.
Take a few steps back from your shelf and just look at the overall shape, the contour. You want to see a varied contour, not one that is straight. If one side looks darker or heavier, move a medium height item to the lighter side before adding a new object. The goal is to have a natural, organic look that makes your eye gently travel across the shelf, rather than creating the feel of a perfectly measured line of items.