Hi, I’ve read 20% of a book whose formatting was quite bad (with a lot of line breaks within every paragraph) and now I’ve found an epub with good formatting, so I’ve uploaded it and I intend to keep reading from this version. Is there any way to move the word list from the previous epub to this new one in order to be able to review all flashcards at the same time? And, if I delete the other book, will the word list disappear with it?
Thanks! 
What you could do is to use the Edit text feature: Edit a text you've uploaded to select the entirety of the imported epub with good formatting, and then paste it into the Document Text field within Edit for the old text, thereby improving the old text. By doing this all your old words will still be linked to the same document as the words you translate going forward.
if I delete the other book, will the word list disappear with it?
When you mark a text as deleted and then proceed to delete it permanently, the words will not disappear, but they will no longer be linked to a text, so you won’t see the name of the text next to it nor will you be able to filter to show/study words just from that text.
Let me know if that all makes sense or if you need more help.