When Do You Need an Agent for Your Book?

Not always. And agents can be challenging to obtain, sometimes almost as challenging as selling your book to a publisher.

You need an agent when you have a book for the general public that you want published by a large mainstream publisher.

The agent will have established relationships with the editors who obtain books for these big publishers. They will know which publishers are likely to be in the market for your kind of book. And they will know how much royalty advance your book will be likely to get.

They will also negotiate the complex terms of your book contract for you. This alone will be worth the 15% commission most charge.

You don't need an agent for a book that is for professional audiences or that will be published by smaller presses. These presses will not give you much of an advance and the contracts they offer will be pretty standard and boilerplate (and since there is not that much money involved in the book ultimately, negotiating that contract is less crucial).

There are also some "trade" publishers (industry jargon for publishers that sell books to the general public) that are small enough not to always require an agent to submit books to them. These are University Presses, and publishers like HCI (who published the Chicken Soup For the Soul Books), New Harbinger (who publish self-help books by psychotherapists mainly) and some others. But even with them, it might be worth your while to get an agent, as they will know what your book is worth and will help decipher and negotiate the complex boilerplate contracts publishers will offer.

How do you find and get an agent? That is a more complex topic and one I will take up in another article.

Hope this helps in getting your book written and published.
P.S. There is a rule of thumb we authors use with agents: The money always flows from agents and publishers to authors. If any agent asks you for money (usually reading fees), run the other way holding on to your wallet. Then search for a real agent.