This book was another excellent book by Kristina.  All of these books do a great job by adding new vocabulary and then reinforcing it throughout the book.   For example, this book has many words heavily reinforced words by nature of the story.   Words such as phantom (при́зрак), dressing room(убо́рная) and trap door / hatch (люк) in various forms are all over the story.   Trap Door itself is in there 24 times in several forms (люк, лю́ков, лю́ки, лю́ком, к лю́ку ) which massively helps seeing how it is used.  For me I had not read the book but did see the Broadway musical but now thanks to Kristina, I understand the story which is an added bonus.  I hope she keeps putting out these books as they are wonderful.