The poem is a small riddle built on its own title. "W-less" tells us a letter is gone, and the last lines explain how: a thief took the final W, apparently to send to his son. The joke is playful, but it carries a tender image, a thief whose crime is not for profit but to give something to his child.
The opening lines set up a contrast. The speaker's books "fit my mind," and the phrase "a music beyond imagination" suggests a deep, almost private harmony between reading and thinking. Then the theft interrupts that fullness with something tiny and odd. Notice that "W" is the last letter of "know" and "write," words tied to books and thought. Take it away and the speaker's world is slightly broken.
The mood is light, whimsical, and a little wistful. The speaker doesn't sound angry. There's a gentle shrug, as if even a loss can be forgiven when it ends up as a gift from a father.