I sympathize.
The "Clear Layout" button should have a dialog that asks "Are you sure?". It certainly clears ALL your design work of the whole book. It has happened to me, so I avoid this button like the plague.
Your only option depends on when you last made a Catalog backup. If it was very recently, then a catalog restore from the backup might bring back some (or all) of your book design, but even better would be-
Selectively restore the Book Collection from the backup catalog by-
1. selecting the Book Collection in the Collections panel with all the photos selected (Ctrl+A)-
(call it "Transfer")
3. then back in your current working catalog- Menu > "Import from another catalog"
4. selecting the Temporary created catalog "Transfer".
This process is similarly described by Victoria Bampton in this link-
https://www.lightroomqueen.com/partial-restore-backup-catalog/
A hint for future work might be to make a "Duplicate" copy of the saved book collection on a regular basis. (Right-click > Duplicate) The "duplicate" will be insurance against major catastrophes like yours. Unwanted Book Collections are easily deleted from the Collections panel.