What are some important quotes from Keats's letters?

John Keats (1795-1821) was an English poet of the Romantic Era who, though not critically acclaimed in the very short span of his own lifetime, came to be recognized as a literary giant, not only for his poetry but also for the ideas expressed in his letters. His letters were published a half-century after his death in 1878 and remained relatively unrecognized in the literary world until the 20th century.
In an 1819 letter to his brother George, who then lived in the United States, Keats writes,

I am certain of nothing but the Holiness of the heart's affections and the truth of Imagination—what Imagination seizes as beauty must be truth—whether it existed before or not.

Here, we see the reason for the driving passion Keats has for capturing in words a transformative but ephemeral moment of beauty—a quest he so exquisitely accomplishes in his poetry, especially in two of the most celebrated and exquisite poems in the English language: "Ode to a Nightingale and "To Autumn."
In an 1819 letter to the poet Shelley, he writes, "My imagination is a Monastery and I am a Monk," revealing the intensity of his devotion to the inner world where thoughts are transformed into the written word.
In many of his letters, Keats deeply examines the very nature of poetry and of the poet, and he comes to the conclusion, in an 1817 letter to his brother George,

that the Poet is the most unpoetical of anything in existence—because he has no Identity—he is continually in for—and filling—some other Body—the Sun, the Moon, the Sea. . .

This quote reveals a great deal about the intensity of Keats's immersion into the natural world, which he depicts like a canvas in words, framed within the highly structured metrical forms that he employs.