The verb “to relish” means to enjoy or take pleasure in something. The speaker expresses sorrow that early death would rob him of the opportunity to enjoy the almost supernatural power of “unreflecting love.”
The noun “garner” is an old term for a granary, where grain is stored after harvest. Keats uses the word “garner” to further develop the poem’s extended metaphor that compares writing to harvesting grain.
The noun “charactery” refers to a collection or system of symbols and characters used to express a writer’s thoughts. In this case, the piles of books contain a collection of the speaker’s thoughts—presumably the ones “gleaned” (harvested) from his “teeming brain.” Keats suggests that these thoughts are so plentiful that they could fill stacks of books.
The intransitive verb “to teem” means to overflow or be full of something. The speaker’s “teeming brain” is full of thoughts, ideas, and feelings that are waiting to be transformed into poetry.
The verb “to cease” means to come to an end. In this context, the speaker’s fear that he “may cease to be” is another way of expressing a fear of death.
