The poem opens with an image of struggle: a mole trying to manage a stone, something far too big and hard for a small, half-blind digger. The speaker compares this to his own life, which has become "under discourse," as if it is being talked over, analyzed, and picked at, perhaps by others, perhaps by his own restless mind. The title, "Discursed," fits that idea of being worn down by words.
The books he mentions are "poor and broken," which suggests that reading and writing, usually a comfort, offer little here. Then comes the last image: oysters opened for a late supper. It's a surprising turn. Books become shells pried open for a small, private meal, perhaps a bit of nourishment found after the hard work of cracking them.
The mood is humble, tired, and quietly wry. The speaker doesn't wail; he shrugs a little. There's weariness, but also a faint sense that something salty and worthwhile can still be found inside the wreckage.