The cover art for their book. You may hate it with a passion; you may think that it misrepresents both the plot and the theme of the book in the worst way possible; you may feel that it reeks of sexism, racism, classism, and other -isms as yet unknown to social science. Complaining to the author will not do any good, because in the hierarchy of people who have a say in a book’s cover art, the author ranks somewhere just barely above the office cleaning crew and the folks in the mail room. With the rare exception of publishing’s 800-pound gorillas, the author’s traditional role in the selection of cover art is limited to bitching about it afterward.
Problems with the printing and typography of their book. If your copy of the book has Chapter 27 replaced by an equivalent number of pages from Love’s Tacky Splendor, that isn’t the author’s fault. Bad stuff can happen to good books when they go to the printer, and somewhere out there is a printing of Love’s Tacky Splendor that has Chapter 27 replaced by a chunk of impeccably-researched hard science fiction. Those readers — and that author — aren’t going to be pleased about this either.
Problems with the sales and availability of their book. The author’s control of these issues is approximately zero point zilch. Finding out that there are no copies of their book to be found anywhere in the entire state of North Dakota is not going to make them any happier.
In all of these cases, as it happens, the appropriate entity to direct your complaints to is the publisher.