When did Geoffrey Chaucer write his first book?

Geoffrey Chaucer did not write a "book" in the modern sense of the word. His works were primarily written as individual poems, not bound together in a single volume.

However, we can look at the earliest known works attributed to Chaucer:

* Around 1370: "The Book of the Duchess" is generally considered his first major work.

* Before 1386: "The Romaunt of the Rose" is an earlier translation of a French poem, but its authorship is debated. Some scholars believe Chaucer translated it, while others dispute this claim.

Therefore, "The Book of the Duchess" is widely considered his first major work, written around 1370.