Wow. This book. READ THIS BOOK!!! It's honestly my new favorite, and I have read a lot of books. The worldbuilding was especially masterful. So many fantasy worlds seem cool at first, but later get boring or simply fail to be original. Caverna is not like that. It is a quirky, dark, unique, and beautiful world that kept me engaged and surprised me until the very last page. The plot is so bizarre and original, and I could never predict where it was going to go, unlike pretty much all other YA/J fantasy books. The characters were so original, fleshed-out, and UN-cliché. Neverfell was so unlike any other heroine ever, and she was so relatable for me! Zouelle and Madame Appeline really kept me guessing, as to where their loyalties lay. The Kleptomancer was a really fun and cool character. The Grand Steward was really interesting too. It was hard to fit him and the Kleptomancer into either the 'good guy' or 'bad guy' category, and those 'in-between' characters are always really intriguing. The writing was poetic and beautiful as well. If you want a dark (but not TOO dark), unpredictable, and completely unforgettable story, this would be the first book I would recommend!
As a side note, I ship Neverfell and Erstwhile so hard right now, while a close second ship would be Maxim Childersin and Vesperta Appeline^^