The nest of the mud-dauber wasps that I found in between my pile of books. Usually I like to crush their abandoned nests but this one is really nice, a work of nature's art, it's beautiful. The most likely species is Asian mud-dauber wasp (Sceliphron curvatum).

Their nests are structurally fragile, that's why i like to crush them with my fingers. Although they are parasitoids (not to humans), they are useful in reducing spiders. But yeah, I try to avoid them as much as possible, just as they avoid me because I'm human.