Fiction is frequently acquired in pairs or larger groups rather than one book at a time. The reason is that the expensive part of publishing a novelist is not the novel.
Launching an author costs more than printing a book
Introducing an unknown name requires sales presentations, retail placement, review copies, publicity and design work, most of which is spent before a single copy sells.
Very little of that expense is recovered by a debut, because debuts rarely sell in the quantities that established names do.
The investment only pays off if the author continues, so a publisher who believes in a writer wants the second book secured before the first one is even released.
Buying two books at once also spreads that launch spending across two products, which improves the arithmetic on both.
The second book is where readers accumulate
Readers who discover a novelist through one book buy the next with far less persuasion. Each release adds to a base that carries forward.
A publisher who acquires only one title risks building that audience and then losing the author to a competitor who bids using the sales figures the first publisher generated.
Tying up several books protects the investment and gives the author a reason to keep working within the same publishing programme.
The author trades money for security
A two-book deal usually pays less per book than the author might command separately if the first succeeds dramatically.
In exchange the writer gets guaranteed income across several years and time to write without repeatedly resubmitting work.
The bet cuts both ways: an author whose first book underperforms is still contracted and paid, while one whose first book takes off has locked in a lower rate.
Option clauses extend the arrangement
Most contracts also include a clause giving the publisher first sight of the author's next book, which functions as a partial extension of the deal.
These clauses vary considerably in how binding they are, and the terms are a normal part of what agents negotiate.
A narrow option covering only the next book in the same genre is very different from a broad one covering anything the author writes next.
What happens when a deal is not renewed
An author whose contracted books sell below expectation can find the record itself becomes an obstacle, since future publishers see the figures.
Publishing under a new name is a recognised response to that problem, and it exists precisely because the trade forecasts from an author's own history.