The most popular book on Project Gutenberg is Frankenstein; Or, The Modern Prometheus by Mary Wollstonecraft Shelley https://www.gutenberg.org/ebooks/search/?query=frankenstein&submit_search=Go%21 . Gutenberg has multiple editions of the book. Your final task is to identify which version here is the most popular one according to their website.

Each final door corresponds to a version, and if you guess the correct door you'll find the center of the maze!