Yep, a book shop in a church.
The internal architecture makes it really quite difficult to photograph this beauty, so just imagine a large church with, instead of pews, three stories of scaffolding which form the upper levels of books.
Selexyz is a stalwart of best book shop lists worldwide, for instance this Guardian article.
Selexyz is a behemoth, and starkly contrasts with the chthonic warrens beloved by many book store aficionados, such as Shakespeare and Company, Paris.
One could argue that the shear size of the place strips back some of the magic of a great book shop, but most things installed in former churches (clubs, climbing walls etc) make for an impressive spectacle and this is no exception. So if you find yourself in Maastricht, which is unlikely on account of it being on the way to just about nowhere, Selexyz is well worth a visit.