This creative set of bookshelves, 5 Dias from Belen Hermoas, was inspired by prison inmates. The shelves are not made out of makeshift shivs or packs of cigarettes, however; instead, they are designed to look like tally marks carved into the wall to keep track of time.
Each shelf comes in the form of five days ("dia" is Spanish for "days"). The shelves can hold up to three books at a time and 5 Dias has a black paint job that slowly turns gray to represent the passing of time. The material that the shelves are made out of is not given, but Hermosa describes it as "lightweight and resistant."
You can see images of Belen Hermosa's 5 Dias here.