The piece that I declined skirted a fine line between the academic and non-academic, doing an excellent job of keeping the manuscript well-cited and fact-based while also keeping a voice that the average person can understand. I admire what they're doing. So why did I decline it?
I declined to edit it because they're no longer at the editing step - they just didn't realize it. They had utilized beta readers as well as a copy editor, and they were looking for someone to content edit. Particularly, they were looking for the kind of content editing a traditional publisher does, regardless of what you've done to your manuscript before it's gotten to them. For anyone pursuing traditional publishing - using a query letter, finding an agent, etc. - especially in regard to nonfiction! - the final content editing is publisher-driven.
These potential clients are definitely not self-publishing, which is excellent, because the quality of nonfiction books on the self-published market is truly lousy. That makes me sad, but if they want their book to go anywhere, they need to go the traditional route. I told them to get the 2015 Guide to Literary Agents, put together a chaptee-by-chapter outline, and have three sample chapters polished and ready to submit (they already do). An agent and a publisher take it from there.
This is very rare - a potential client who does not in fact need the services I require - but when it happens, I am (and always will be) up front!