That really depends on your book, its structure and intended audience. But let’s keep it simple.
Typically a book requires the following services:
Editing: So that the book reads well, and is free from grammar and syntax errors.
Cover Design: So that the book is nicely presented and has all relevant information.
Page Design (or Interior Layout): So that the book reads well, has a choice of fonts appropriate for the target group, a nicely designed table of contents, and copyright and all other such information.
That’s it. That is actually all the book needs to gain a physical presence or be accessible to the buyer. Further, though, there are other services you may wish to consider:
Illustrations: Something you may want if it is children’s book, or if you wish to drive home a point with an illustration.
Indexing: Mostly used for academic texts, available for both, subject and person.
Marketing: Digital marketing services that will help your book capture the market’s attention.
Distribution: So that your book is accessible within domestic and international markets.