This email exchange with my managing editor pretty much sums it all up:
ME to me: I meant to mention to you that Yada Yada would like to include the extra information (and credit you) that you added as an "FYI" to him in note 83 of chapter 10 of Blah Blah Blah (about the whatchamacallit being used in Ringydingy in the 1970s). Is this OK with you?
Me to ME: That credit belongs to Whats Hisname.
Whats Hisname, an editorial consultant who occasionally eyeballs manuscripts, added one sentence to the book. I made well over a thousand edits, many of them substantive.
I will receive no credit or thanks of any kind from the author (nor will the layout person who transforms the Word docs into a publishable PDF file in InDesign), and certainly no mention in the book's Acknowledgments section. That little perk always goes to the acquisitions editor who brought the manuscript in and then, with virtually no review, turned it over to Editorial. Every time I read that section of a manuscript and see that acknowledgment, I want to scream.