The speaker wants his friend to stop reading his books and head out into nature. This isn't so much because he wants him to take time out to smell the roses, as it were; it's more that he thinks he'll learn so much more from the great Book of Nature...
The speaker wants his friend to stop reading his books and head out into nature. This isn't so much because he wants him to take time out to smell the roses, as it were; it's more that he thinks he'll learn so much more from the great Book of Nature than he will from the books on his desk.
As an arch-Romantic, the speaker endows Nature—note the capital N—with almost superhuman qualities. Yes, it's nice to go out into the forest and hear the birds singing, but there's so much more to Nature than beauty; it's a living, breathing force. More than that, we can learn so much from it. In a rush of hyperbole, the speaker maintains that there is more wisdom in the sweet music of the song of the woodland linnet than in any books one might read.
And the throstle is not just a bird; he is a "mean preacher." He has something important to say as he twitters loudly on the branches. The throstle, like the woodland linnet, acts as a corrective to what the speaker describes as the "meddling intellect," or reason that often dissects and objectifies Nature and, by doing so, murders it.
What Wordsworth is criticizing here is the scientific attitude to Nature, which turns the environment into nothing more than an object of exploitation. He recommends a radically different way of conceiving our natural environment. Instead of dominating and controlling it, as the speaker's friend's books might well recommend, we should learn from it. We should get out into the open air and listen to the voice of Nature, as expressed in the song of the throstle and the wood linnet. In doing so, we will become wiser.