In "An Acre of Grass," Yeats' speaker is an old man who believes that books are his "acre of grass." Because he is an old man, he can no longer run around outside to keep fit; his "strength of body" has left him. However, his mind has not. At the end of the poem, he calls it "an old man's eagle mind." He's no longer able to keep his body sharp and strong, but he can keep his mind in top form.
In illustration of the books he turns to, Yeats mentions the great literary artists. For instance, he mentions, King Lear, one of Shakespeare's most tragic figures. He mentions the literary giant William Blake and the great painter Michelangelo. Through these minds, which he can still read about and which can still inspire him, he is able to keep his mind "inspired by frenzy."
Symbolically, the old man desires to become these different people. It's not enough to read about them. In the case of King Lear, I would imagine that the old man would want to learn from Lear's mistakes and continue on to "make the truth known." The advantage he has as an old man is wisdom. That is where his "eagle mind" will ultimately help him. Coupled with the "frenzied" motivation of time running out, he can still become the person he always wanted to be.