A capacity, and taste, for reading, gives access to whatever has already been discovered by others. It is the key, or one of the keys, to the already solved problems. And not only so. It gives a relish, and facility, for successfully pursuing the unsolved ones.

Lincoln is praising reading as both an intellectual tool and a cultivated pleasure. To him, the habit of reading opens the accumulated store of human knowledge—“whatever has already been discovered”—so that a learner need not reinvent solutions to problems already worked out. But he also stresses reading’s forward-looking power: it trains the mind, builds vocabulary and reasoning, and gives “relish” (motivation) and “facility” (skill) for tackling new, unsolved questions. The quote reflects a distinctly self-made, democratic ideal of education: access to books can substitute for formal schooling and can equip ordinary people to participate in progress.