The Copyeditor's Handbook by Amy Einsohn is organized well, from the most common problems such as comma placement and hyphenation to how to edit tables and the index in a nonfiction book. Each chapter ends with an editing exercise or two, and the answer key provides clearly stated reasons for the editing choices, along with most possible alternatives.
The exercises were great: not only did they practice the subject of the chapter, but they gradually built up in difficulty. As an example, the first exercises were fairly mechanical, only dealing with punctuation. By the end of the book, you would have to pay attention to everything else that had come before and the topic of the chapter. So, even if the instructions asked you to pay attention to wordiness and how numbers are treated while making sure that the author's personal style was not messed with too much, you also had to remember to check punctuation, dangling participles, and run some fact-checking ("Is the tennis player's name really spelled like that?") Pretty neat. Also, there are some silly jokes embedded into the example sentences. Kind of reminds me of my Finnish syntax classes back in the day, where the teacher would, with a totally deadpan expression, give example sentences such as "The meat is infested with maggots." I guess her method worked, because I still remember that sentence very, very vividly.
Also, I have been reading two other books within this same copy editing topic, and I gotta tell you.... There's no grammar book, or a style manual, that doesn't quote the famous Star Trek line as an example of the old, silly rule of thou shalt not split your infinitives. Don't make me write it out; you know what I mean. Nerd.