It's important to realise that you cannot import PDFs, Word or Google Docs or any other type of files.
Even other apps that create ePub files, such as Apple Pages or iBooks Author, won't work if you try to import them into Book Creator.
The most import use for importing books is if you want to manually transfer books you've made in the Book Creator for iPad app to your online account, or vice versa.
Importing a book
Click the Books button beneath any book on the home screen.
Choose Import book.
If the book is on your computer, choose Upload from computer, or simply drag and drop the ePub file into the browser. Or choose Import from Drive and navigate your Google Drive folders to find the book.
You’ll see the book importing and will get a message when it’s done.
Importing books in Book Creator for iPad
On the iPad, the process is the same - tap the Books icon, choose Import, then find the book you want to import using the Files app. Make sure you've connected any cloud apps you're using to the Files app - here's how you do that.
Note, you'll also see a 'Copy from iTunes' option - this option was applicable to older versions of iOS that connected to iTunes. Newer iPads don't connect in the same way, and the Files app has superseded this method.