This quote uses the metaphor of a musical instrument to describe the influence and impact a good book can have on us. In this context, a "lyre" represents our minds or souls, which, like a musical instrument, have the potential to produce beautiful, profound outputs. However, these potential outputs often remain silent due to lack of stimulation or inspiration. A "plectrum" is a tool used to strike the strings of a musical instrument, such as a guitar or lyre, to produce sound.

So, when Thoreau refers to a good book as the "plectrum," he means that a good book has the power to stimulate our minds or souls, to provoke thought, emotion, and reflection, and to bring out the latent potential within us, just as a plectrum brings out the latent music within a lyre.

In today’s world, this idea is more relevant than ever. In the age of digital media and information overload, our minds are constantly bombarded with shallow, fleeting stimuli that often leave us feeling numb and disengaged. A good book, on the other hand, offers a deep, sustained engagement that can awaken our minds, stir our emotions, and inspire our thoughts in a way that few other things can.

In terms of personal development, this quote underscores the importance of reading good books as a means of self-education and self-improvement. A good book can broaden our horizons, deepen our understanding, challenge our assumptions, and stimulate our creativity. It can help us to see the world from different perspectives, to empathize with different experiences, and to reflect on our own beliefs and values. In this way, reading good books can be a powerful tool for personal growth and transformation.