How To Read A Publishing Contract
Five clauses determine what happens to your book for decades.

A publishing agreement is long, and a small number of provisions determine nearly everything an author will care about years later.
The grant of rights
The first question is what is being licensed: which formats, which territories, which languages, and for how long.
A grant of world rights in all languages is different from a grant limited to English in defined territories. Retained rights can be sold separately by an agent, and for some books that is where the money is.
Reversion
Establish the conditions under which rights return to you. Historically this depended on a book being out of print — a concept that print-on-demand and digital editions made close to meaningless.
A modern reversion clause should be based on defined sales thresholds over a defined period, with a clear mechanism for the author to request reversion. Without it, a book can remain nominally in print and commercially inactive indefinitely.
The option clause
Many agreements give the publisher an option on the author's next work. Narrow it: to a single next book, in the same category, with a defined period for the publisher to decide, and without the right to match another publisher's offer.
A broad option clause can tie an author's entire career to a relationship that may not be working.
Acceptability
The advance instalment on delivery is usually conditional on a manuscript being acceptable to the publisher. Where possible this should be tied to editorial standards with an opportunity to revise, rather than resting on unqualified discretion.
Accounting and audit
Statements are typically issued twice yearly with a lag. Reserves against returns can be withheld, and the clause should limit their size and duration.
An audit right allows examination of the publisher's records. It is standard, rarely exercised, and worth retaining.
The advice
Have an agent or a lawyer who specializes in publishing review it. These agreements last for the life of the copyright, and the terms are negotiated once.