Emily Dickinson's poem entitled "He ate and drank the precious words" comes from her collection of poetry under Part One of "Life". It is number twenty-one in the collection.
Please remember that with any poem, the interpretation belongs to the reader. Therefore, the answer that I provide is based upon my own personal interpretation of the poem and its theme.
The poem refers to a man who is far past his prime. The man is metaphorically drunk off of the words that he is reading:
He ate and drank the precious words
His spirit grew robust
After, the poem speaks to the fact that the new knowledge brought to the man allows him to forget that he is poor and his fame has been forgotten by all:
He knew no more that he was poor
Or that his fame was dust
At the end of the poem, the man is so intoxicated (by the words he is reading) that he is able to feel free from any concerns which he may have had prior:
What liberty/ A loosened spirit brings.
The theme of the poem is freedom. The theme emerges from the fact that the man in the poem is able to break free from his past based upon what he is reading.
One could look at the book as being something spiritual (like the Bible) given the words in the book freed him from his past.