restore my iPhoto library

Assuming you have a backup of your iPhoto library you can, you have to revert to an earlier operating system. Of course, you won't be able to print your books through Apple, but you can get them back.

Revert your Mac to a previous macOS version - Apple Support

I suggest you just move on.

This the Printmaster you mean? Printmaster 8 Platinum?

https://apps.apple.com/ie/app/printmaster-8-platinum/id1253146180?mt=12

Did you read the first line of the description? Right there were it says...

This application is not compatible with Mac OS 10.15 Catalina

If not, what app are you referring to? And what's the exact error message you get?

The only way you can do what you want is to restore your Mac back to what you have just prior to upgrading to Catalina. That will let the iPhoto version you had run but you will not be able to order books like you used to.

To order a book hold down the Option key and click on the Buy Book button.  That will give you a PDF file that is the same as was uploaded to Apple when they were still offering print services.

There's a 3rd party book provider which used to do the printing for Apple as a sub-contractor, Mimeo, that will take PDF files created by iPhoto and Photos and print and bind books and calendars. 

Also you can upload the PDF file to PrestoPhotos which can also print and bind books and calendars from iPhoto or Photos pdf files.  They've been doing it for some time now.  

Use the PrestoPhotos or Mimeo links above for direct access to the PDF file uploading and book/calendar ordering page for each site. 

What I've done is restored my Mojave system to an external SSD

so that I can run the last version of iPhoto, 9.6.1, to create books and calendars and order from Mimeo. Their books are essentially the same as Apple's were as they were a subcontractor to Apple. The only difference is they substitute their logo on the back for Apple's.

Since Apple no longer offers the "Apple Inherent Product", there is not much you can do.

Are you on Catalina?