Sorry if its obvious, but which of Marion's books is he talking about?

Like I say, it's a snap theory and almost certainly wrong, but I think it might be an interesting way to bring back the Time Lords given available evidence...

What if Susan is now the President pro-tem and is hiding somewhere with the remnants of Time Lord, hidden away during the Master's destruction of Gallifrey. Now they're in trouble, and Susan's only resort is to call upon her Grandfather, possibly explaining "Find me"

There are two people in-universe who wear white in a characteristic way; one is the White Guardian, the other is the High President of Gallifrey...I focused on the latter.

Quick one-off theory I had that's probably wrong, but I'd like to get it out there anyway.

So, the vision in the ep shows Susan in the TARDIS wearing a long white gown. While it could definitely just be a coincidence, that white gown made me wonder