I was searching for an answer to this problem…apparently, it's not easy to find iBook in the list of apps where you can share a book. All the solutions I found said to open Dropbox and select the file you want to share. But, then, they tell you to tap the ellipsis (…) in the upper right corner but that didn't work. You need to tap the ellipsis that's next to the title you want to share or export. Then, choose 'Save to Device' choose 'More' and you should find Books in the list of options. Hope this helps others…and hope I can find it next time. 😁
