What is wrong with this part of the sentence: "he worked as a writer, and editor, and a library"?

The issue with the phrase "and a library" is that "library" does not fit within the parallel structure of the list being presented. The phrase begins with "as a writer" and "as an editor," both of which are professions. The use of "library" does not align with the context since it refers to a place, not a profession or role. Therefore, the sentence lists two job titles (writer and editor) but then improperly adds a noun that does not match that pattern.

In contrast, the phrase should maintain consistency by listing roles or titles. A more fitting term in that position might be "a librarian," which would complete the list in a coherent way, ensuring that all items referenced are professions. Keeping a consistent theme is crucial for clarity and coherence in writing.