If that's you, talk to your editor. You might need a contributor or co-author.
ContributorA contributor is someone who provides content on a fee-for-hire arrangement. If you want someone to write a chapter on, say, arterial blood gases, you'd ask an expert in that topic to supply a chapter in return for some remuneration, the amount of which is up to you and the contributor to agree on.
Contributors are paid when the book publishes, to avoid situations in which contributors are paid but their content is never used or the book never publishes. They're paid once and that's it.
Contributor payments come directly out of the author's advance, so the more a contributor is paid, the longer it will be before the author receives royalties on the book. Common amounts paid vary from a couple hundred dollars per chapter to a thousand or more, depending on the level of expertise of the contributor, the size of the chapter, and the anticipated amount of work required.
Co-authorA co-author, on the other hand, is someone who shares in the royalty agreement with the author and publisher. Co-authors are expected to do much more than contributors, and so are compensated at a higher level.
They generally receive no money up front, unless so stated in the Author Advance clause of the contract. Co-authors, like the main author, are compensated through the sale of their book.
Choose wiselyIf you choose the co-author road, make sure you choose someone you can work with for a long time. Contractually, author teams can't legally break up without the mutual consent of all parties involved. Most of the time, author teams form because each member already knows, respects, and likes the other members, and few problems ensue.
However, sometimes that's not the case and people who get together for the first edition grow to dislike each other or otherwise drift apart. Hey, that's life.
So if you bring on one or more co-authors, make sure they're people you absolutely can work with.