Also trying to understand:
I believe that the Book module does not have page 'Presets' as such.
You can design "Auto Layout" presets and these will be in a folder "Auto Layout Presets". This folder will be in your [Your catalog/Lightroom Settings] if you store presets with your catalog. If you have changed the preferences "Store with..." option at any time you will find this folder in both places- Your catalog & App Data.
Custom Pages do not appear in a folder anywhere, but exist 'hidden' within a "Saved Book" Collection or within the "Auto Layout Presets" folder. Custom pages seem to only appear in the Book where they were used. So it would seem that if we want to re-use custom pages in new books, we should save an "Auto Layout" preset while we have all our 'Custom' page designs available in the current book design.
Does all that make sense?
Forgot to add-
Custom Pages only exist in one Page size format. eg. custom pages in a 10x8 book will not appear in a 12x12 book.
Thanks for the detailed explanation.
I found another thread here A method to create completely customized photo book page templates from scratch in Lightroom 5 which says that the book layout templates are stored at %PROGRAMFILES%\Adobe\Adobe Photoshop Lightroom 5.2\Templates\Layout Templates" and I have found mine in the equivalent LR CC folder; interestingly, there are no other templates in this "Templates" folder, just a folder full of book templates.
So, it seems that the book layout templates all ignore the preference selection to "store presets with this catalogue".
Thanks for the link, Metro_Mike.
Looks too deep for me!! Scripting and LUA are beyond my capacity. Keep on the chase.