[nonethewiser]

People believe different things about the Bible. She me people think swearing on it is making a promise to God.

Does the person swearing on Sagans book think Sagan is going to hold them accountable to a broken promise?

[sokoloff]

I conclude that person thinks Sagan will hold them equally as accountable as God would if they’d chosen His book.

[yetihehe]

Yes, I think it's equivalent and Sagan will held them as accountable as your chosen god. For "the person", you can just ask directly.

[prawn]

They might be doing it intending that they will remember and endeavour to hold true to some message of the book.