[Bookpossum]

I'll second Lyrical Ballads.

I would like to nominate T S Eliot's Four Quartets. From Kobo:

Four Quartets is the culminating achievement of T.S. Eliot's career as a poet. While containing some of the most musical and unforgettable passages in twentieth-century poetry, its four parts, 'Burnt Norton', 'East Coker', 'The Dry Salvages' and 'Little Gidding', present a rigorous meditation on the spiritual, philosophical and personal themes which preoccupied the author. It was the way in which a private voice was heard to speak for the concerns of an entire generation, in the midst of war and doubt, that confirmed it as an enduring masterpiece.

Not very long (about 35 pages in my paper copy) but very deep and immensely rewarding. For those who do not know this work, here is a small sample from The Dry Salvages:

For most of us, there is only the unattended
Moment, the moment in and out of time,
The distraction fit, lost in a shaft of sunlight,
The wild thyme unseen, or the winter lightning
Or the waterfall, or music heard so deeply
That it is not heard at all, but you are the music
While the music lasts.