How do I export/import ebooks in Bluefire Reader for iOS?

To import ebook files, tap on the "+" button in the Library view. Navigate using the Files App that pops up to EPUB or PDF files and tap on "Open"

To export files, tap "Select" in the Library view, then choose the books you wish to export, then tap on the "Share" button which will launch the Files app and you can navigate to your desired storage location. 

I highly encourage you to back up your books using this export method, and choose a cloud storage service such as iCloud so that if your device is ever retired or lost, you can import you ebooks onto a new device.