Visibility asks whether people can encounter you. Discovery asks whether the right people can find you.
Specifically, can they find you at the moment they need exactly what you created. Visibility is about presence. Discovery is about timing and intent.
A reader browsing a genre list is visible territory. A nurse educator searching for a handbook to recommend to a cohort, a conference programmer looking for someone to speak on a subject, a training manager assembling a reading list, those are discovery moments, and they rarely begin on a retail page.
Discovery work maps those moments. It asks which audiences exist beyond readers, what each one is trying to solve, where they look, and what they need to see in order to recognise the book as the answer.
Who might need this book, other than a browsing reader.
The discovery work I handle. Research that establishes where a book belongs, followed by the pages and pathways that make it reachable there.