Technically, proof-reading happens after a book is formatted, and it’s the final step before you hit that ‘publish’ button.
This is the very last round, reading the proofs (or galleys) with the book in its bookish state. We’re looking for any typo that valiantly made it past the copy-editing* stage. Hate proofing? I’m on it!
This kind of activity makes me do happy dances.
(Easy Math for Writers: $3 per 1000 words)
*looking for a copy-edit instead? You want this page.