This copy turned out to be in good shape, other than a bit musty, but it's an MMPB instead of a trade paperback, so they don't match.
Kim and Mairelon have moved to his brother's house in London where Mairelon's aunt would like them to be more acceptable. She's going to make sure Kim wears girl's clothes and has a chance at getting married. Kim is not so interested in that. Someone tries to steal a book from the house's library, which sends them into a lot of study and analysis, more as it happens again and again. There's some kind of uncommon wizard around who keeps attacking not only the toffs, but the street wizards.
Mairelon's mother, Lady Wendall, arrives with a monkey (present from a friend) and decides to give Kim a Coming Out Ball. Kim is still not interested, but for various reasons, she agrees. There is magical disaster and romantic fulfillment by the end of the book. I really enjoyed this series.