What is a noun that can replace long work of fiction?

Here are some nouns that can replace "long work of fiction," depending on the context:

Formal:

* Novel

* Epic

* Saga

* Magnum opus

* Masterpiece

* Literary work

* Prose work

Informal:

* Book

* Story

* Tale

* Narrative

* Fiction

Specific Genres:

* Fantasy

* Science fiction

* Romance

* Thriller

* Historical fiction

More creative options:

* Literary tapestry

* World-building

* Imagination

* Dream

The best replacement will depend on the specific context and the intended tone of your writing.