Pages 10.3.9 won"t import document

Are you inferring that even Pages for iCloud can no longer open these "invalid file format" Pages documents? Very strange, that it would mark the file(s) that way. Have these Pages documents been stored on any other cloud service than Apple's iCloud?

On your Mac, and in System Preferences > Apple ID, click that icon to see how much available storage is in your iCloud Drive account.

When one has been working on a book for decades, it would seem that the importance of the document would mandate multiple backup copies, whether via Time Machine, and/or other external storage resources. Not having some form of backup strategy pretty much eliminates any urgent advice.

Pages autosave commences once you formally save the document. It is not the only backup you need as once the document is damaged, or Pages thinks it is, you are done with that document. Without a formal external backup to refer to, whether in the form of a Time Machine backup or just retrieving a file copy from iCloud, or a USB stick — you have no means to recover the current document, and will be required to reconstruct it from the parts that you do have.

What is puzzling to me is Pages telling you that it cannot import a document because it was in an invalid format. The current Pages can open (there is no formal import) Pages '05 through Pages v10.3.9 documents, IBA, RTF, RTFD, DOC/DOCX, and text. It cannot import AppleWorks documents or any other document format. Unsupported file types are simply unavailable for selection in the Open file dialog.

A Pages .pages document cannot be opened by any other application on the planet. Why are you having friends attempt to open it in Microsoft? All MS Windows will do is think it is a zip file and unzip it. No MS Office application will open it. That act alone would render the document an "invalid file format." Same story with double-clicking a Pages document on Google Drive, if they were to attempt that.

You can ask the Apple representative to escalate the issue or let you speak to their manager. However, if that Pages document was damaged via the first paragraph, there will be no be no means to undo the document damage.

As someone who used to write for a living (semi retired now) I certainly sympathize with your awful plight. I have never written a book but for me losing even a 20 thousand word article would be devastating, let alone an entire book. I don't know if the Apple Stores have reopened yet--or if there is one near where you live-- but if I were you I would take my computer to an Apple Store if that is possible and see if one of the "Geniuses" can find a way to recover your book. That said, VikingOSX is, sadly, quite correct both in a technical and personal sense. Saving your work --whether in draft or editing stage-- to multiple sources-- via the cloud and external drives on a daily, even hourly basis--is an essential safeguard against work disappearing, possibly forever. I have never fully trusted iCloud and use two third party storage providers in addition to iCloud. Depending on the assignment, in the past I have occasionally used Google Docs to write, particularly when traveling but also sometimes at home, since I know that no matter what happens while I am composing or afterward--disaster strikes, my computer dies or gets stolen (happened once when abroad), the house burns down, etc.--I will never lose anything written. When a final version is completed I download it as a Word Doc or PDF then print or send it as an attachment to the publisher. I very much hope that someone can come up with a solution for you. Sorry that I couldn't be of more useful help.

As I wrote before, I am in full sympathy with your plight and the frustration, anger and disillusionment you have been experiencing. Is the book you were working on fiction or non-fiction? You wrote in your most recent message, "I am loathe to begin all over again, as what is to say that the same thing won't happen? If you can't trust Pages, or iCloud, or Apple, what do you do?" One thing you might consider in the future is using one or more of the third party cloud storage services as suggested here by other posters. Backblaze and iDrive are I think the most widely employed by Mac users. You can set up either of them to backup all your data (not just documents) continuously as you are composing or incrementally at certain times of your choosing. These are not free apps but require a monthly or annual subscription (Backblaze I believe does have a free tier with 5G total storage). As mentioned before, Google Docs is another alternative. If you no longer trust Pages to save your work (understandable) there are many other options including LibreOffice which is cross platform (versions for Mac, Windows, Linux) and includes a very good word processor as well as spreadsheet and presentation components. I know this is small comfort but you may have read at some point about what happened to Hemingway when he was 23 years old and living in Paris with his first wife Hadley. Hemingway was in Switzerland on assignment (still working part time as a journalist then); when his wife was boarding a train at Gare de Lyon on her way to meet him a suitcase containing much of what creative writing Hemingway had done up to that point in his career was stolen and never recovered (Hadley was bringing it to Switzerland as a "surprise" for her spouse--along with the carbons! Which made no sense). Needless to say, Hemingway was not exactly happy but two years later a small collection of his stories was published and two years after that "The Sun Also Rises" appeared. Easy for me to say but you wrote that you still had prior versions of individual chapters and maybe you can reconstruct your book--a daunting task I know.

Great commiserations, Casuarina,

A few times I have retrieved small, ordinary document-type files that "could not be opened" by the software in use.

My method was unconventional, but certainly proved worth my while. (I haven't tried it since Catalina + Big Sur though.)

I would first "copy" the file, and leave the original untouched at all costs.

Then I would try changing the file extension on the copy to something else, (eg: .rtf .doc. .txt)

I would "ok" the warning that it might leave the file unreadable, since I had nothing to lose.

Then try opening the file in every suitable software that I had., "Text Edit" included.

Sometimes I would send that copy to the bin, and try a different file extension.

A few times, "Voila" here was my original text! I could then copy and paste it into another document for safe-keeping.

All the very best to you.