What is bookmatching?
One of the best features of natural stone is the innate variation in each piece. No two marble countertops are ever the same, and therefore no two kitchens with said stone are ever the same. It's a classic way to add a unique element to your design, and the fun part is going to the stone yard to pick out your slab!
That being said, slabs that are cut from the same stone often look alike, and can be made to look almost identical through a process called bookmatching.
Imagine a large marble boulder, then slice it in half, and polish each flattened side. When opened (like a book), the polished, flat sides would essentially be mirror images of each other. When sliced, the veining and color variations that flow within the stone would match on either side. Not exactly identical, but close enough to appear so. This is bookmatching.
These walls each feature two bookmatched slabs.
With the right stone, you could repeat this mirrored look with four, six, eight, etc. slabs next to each other, although I do think the symmetry can be overdone. That might work in a swanky bar or office building, but in my opinion, not a home.
I think vertical applications are the preferred way to showcase this style, though bookmatching could certainly be applied to a countertop or flooring or however else you can imagine, and the effect can be created out of other materials as well, such as these bookmatched wooden tabletops designed by George Nakashima.