The house may well tell you to make any queries to author in the form of margin comments. I never do – I place them directly in the text, <between angle brackets, highlighted in blue>. Obviously, your queries should be brief, precise, and respectful. Also, they should be cast in a neutral tone – no overfamiliarity or drollery – since you simply can’t know (except from what little you can pick up from the author’s background and writing style) what attitude the author is going to take toward you. Some authors are primed to assume that you’re wonderful no matter how little proof of that you provide. Others will assume from the start that you’re there to be kicked. There are ways to sell the author on your expertise, most obviously by pointing out factual errors you’ve caught. (<Gerda Taro wasn’t hit by a truck and did not die instantly. She was run over by a tank and died that evening> [caught you, ha-ha]). Also, by keeping your own typos to a minimum. You will always leave a few of your own, so spell-check before you return the mscript.
Now you scroll down the file you’re about to send the author, wondering how “coloured” it’s going to look. A heavy edit would colour up more than a light edit, obviously. It’s hard not to be a little nervous, because the last thing you want to do is shock the author. Granted, a copyedit is supposed to sting a little, but you hope the author’s gut reaction will not be, “Oh my God, what the …”
And that file is what you send, with a cover letter to introduce yourself, tell the author what you found, and tell him what to do next. The same rules apply as to your comments: be brief, precise, and respectful, and avoid overfamiliarity. Tell them you enjoyed the project, whether you did or not. Don’t go into any detail at all about what you actually found – he’ll notice soon enough. Provide separate paragraphs for the textual work you did and for the documentation. And don’t explain yourself.
When I’ve written a longer letter to the author, I’ve always cast it as a standard cover letter plus memo. Only twice have I felt a need to do that, both times with regard to documentation. (“Re documentation: Regarding the documentation issues, please read the attached memo carefully”). On one occasion, it was an author’s first published mscript, and he was thorough to the point of madness. It was a 900-page monograph on Positivist philosophy, and more than half of it was doc. And the doc, I noticed soon enough, besides being multilingual, included every occasion where he had encountered a source. One wasn’t good enough. And if he encountered it in three or four different languages, he provided each one of them. If he found it in three different articles, or six, he listed all three, or six, citing each one in full. Also, he never short-cited anywhere, even it was obviously called for. Did I mention that this all happened during pencil-edit days? Then he handed it over to a grad student to type up, who it turned out was ESL Chinese. I recall starting a random check on the biblio and how it dawned on me, with mounting horror, that not a single one of them was remotely accurate. And I recall the memo I sent him, which began, “You need to brace yourself from some criticism,” and boy did he get some.
That was back in pencil-edit days. One of my last projects was theatre history, and the author provided a 300-page biblio. By the time I was done with it, it was three pages. And the 250 pages of notes? It was down to 70. But that’s for another day.