The Yellow Phantom
This is another Judy Bolton book. The main plot centers on some poetry that has been written and apparently some of which has been stolen. Judy is the main character but there is also Dale, who is a mystery writer, Irene, a young girl and Emily Grimshaw, a book agent that hires Judy to help go through some bad poetry to get it into proper shape to be published.
Some poetry goes missing and so Irene herself goes missing. There's also a major villain, of course. There's an old tower that becomes important and Irene's background is cleared up. It may be an old story but it's still a good one.