This is a simple book end, for lightweight use. PLA tends to bend, over time, so you should not rely on this to hold up heavy books and objects; but it should suffice for basically keeping things upright.

The holes are there to save on filament, though it slows down the printing process. To strengthen this as a book end, you could add triangular supporting structures to the back, but then you could not stand it right at the end of a shelf.