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Are there any novels, short stories, or fluff background pieces that feature Space Marine Dreadnoughts, either as the main protagonist or as a prominent cast member who receives a lot of focus?

Short stories:
Iron Soul, Iron Hands dread
Penumbral Spike, Obsidian Glaives dread
The Relic, Black Templar dread

Novels:
Deathwatch, Lamenter dread
Battle of the Fang, Space Wolves dread

Comic series:
Damnation Crusade, Black Templar dread

Audio:
The Glorious Tomb, Black Templar dread

There's plenty of others that feature dreadnoughts but these definitely have them front and center.

While they don't take up the bulk of the pages, the Dreadnaughts steal the show in The Emperor's Gift and Soul Hunter, both novels by Aaron Dembski-Bowden. The latter is potentially the most realistic, gruesome, horrifying portrayal of what it actually means to be a Dreadnaught I can think of in 40k.

Audio Drama:
The Glorious Tomb. War from an entombed brother's perspective.

Sound and Fury, which appears in Ghostmaker, features a Chaos Dreadought as its primary dueteragonist.