If you already follow Epic’s light switch tutorial, the next step would be “blueprint communication” tutorial.
Which you still accept event ‘E’ for a book setup like a light switch, but instead of doing anything, you call a custom event that defined in your secret door BP that opens itself.
Where do you get this relationship then? you can save a reference by create a variable that is an instance(object with _C in name when you choose variable type) from your secret door BP.
OR, if it’s only level specific, you can use event dispatcher and BIND the book’s event to secret door’s open event.
Go check the tutorial if above is not clear to you, as you would need those knowledge to understand.