To be clear from the outset, Stella is not the “fairest book of nature” herself, but she is certainly a very important part of it.

The speaker of the sonnet invites anyone who wishes to observe a fusion of virtue and beauty in nature to look no further than Stella, who manages to combine both these qualities in a way that is truly unique.

In the Elizabethan era, beauty was widely believed to lead to virtue. And so one can see that observing Stella's beauty is so much more than just contemplating a pretty young woman; it also has a moral dimension. What makes Stella particularly special is that she herself manages to combine both virtue and beauty.

In the shape of this truly remarkable woman, this epitome of beauty and virtue, all vices have been overthrown. So much so that the very contours of her exquisite figure show true goodness. What's more, Stella doesn't extirpate vice by “rude force” but simply by virtue of the “sweetest sovereignty / Of reason.” Her beauty draws men's hearts towards her, and her virtue turns their love to good.

Even so, in a truly remarkable twist right at the end of the poem, the speaker suggests that that's not enough, that there's still a need for “food,” meaning that desire must still be satisfied.