Well. If I'm doing it, even a level wouldn't help. :-D
You don't need it.
You will want to make the upright portions the same length, and the grooves for the shelves at evenly spaced positions on them. This only requires that you put them side-by-side and mark them.
What may be of benefit is a carpenter's square, although a small version is adequate. Even this can be done away with by careful measurement.
Mostly carpentry is geometry, and the rules for one apply to the other. Mark your boards in parallel, and assemble them square by measuring the diagonals as equal. Fasten a backing sheet of plywood when they are square. It will remain square when you stand it up.