My question is about nonfiction books, which make up many of the query letters you recieve. How do nonfiction queries differ from fiction queries? Is the publishing process the same?
The queries are still a basic one-page deal, but the author usually spends a lot more time discussing their credentials to write this book than they would in a fiction query. Where it starts to differ is when the agent asks for more material. In fiction, it'll usually be a couple chapters and maybe a synopsis. In nonfiction, it will likely be a proposal, which is a long and formal thing discussing chapter content, source material, credentials, and discussion of marketing possibilities in comparison to other books on the market with the same topic.
Every once in a while a fiction author sends material in the format of a nonfiction proposal - discussing all the things I mentioned above. This is totally ridiculous. We just want to see your writing. Send that unless the agent specifically says "I want a proposal."
The publishing process is generally the same, except the advances are generally higher in non-fiction but the editing process is much longer, as it often involves checking source material and getting permission from other media to use their pictures/content.