Do you have iBooks Author (and your projects) backed up to a TIme Machine drive from which you can restore them? Another backup of your .iba files on different media?
I hope you realize that Pages v10.1 and later are not iBooks Author by another name, and substantial .iba content, depending on what you have done in it, may not map that functionality to Pages when the .iba files are opened in it.
Your published books at the Apple Books store were in either in PDF or EPUB format, and neither can be reverse-engineered to a .iba document format, nor opened by Pages. See the link on this Pages below Related Article.