Novels are usually sold complete. Non-fiction is usually sold as a proposal, with a couple of sample chapters and an outline standing in for a book that does not yet exist.
The research is the expensive part
A serious non-fiction book can require years of archives, travel, interviews and reading before it can be written at all.
Asking an author to fund that entirely on speculation would restrict the form to people who can afford several unpaid years.
Selling on proposal converts the publisher's advance into working capital, paid in instalments that carry the author through the research period.
Payments are typically staged across signature, delivery and publication, which keeps the publisher's exposure limited while still funding the work.
A proposal is a business document
It sets out the argument, the structure, the intended readership, the comparable titles and what the author brings that nobody else could.
Sample chapters demonstrate that the writing will be readable, but they are only part of what is assessed.
The proposal is judged largely on whether an audience for the subject can be identified in advance, which is a marketing question rather than a literary one.
Platform weighs heavily in the decision
Because the book cannot be evaluated as a whole, publishers lean on evidence about the author: their standing in the field, their existing readership, their ability to attract attention.
This favours academics, journalists and practitioners with established visibility, and disadvantages outsiders with an unusual subject and no public presence.
It is the main reason two writers with equally good ideas receive very different responses.
The finished book can diverge from the pitch
Research changes arguments. A book written honestly over several years may reach conclusions the proposal did not anticipate.
Editors generally expect some drift and work with it, though a manuscript that no longer resembles what was bought creates genuine difficulty for a publisher who scheduled and positioned the original.
Contracts address this with acceptance clauses allowing a publisher to reject a delivered manuscript, and the terms governing repayment of the advance are heavily negotiated for that reason.
What the system produces
Selling on proposal makes ambitious, research-heavy books possible by funding them in advance, which is a real benefit to readers.
It also pushes non-fiction towards subjects that can be described persuasively before they are understood, and towards authors whose names already carry weight.
The books least served by the arrangement are those whose value only becomes apparent once the research is done, which is a category no proposal can convincingly describe.