treasures to/for

I would use them to describe slightly different relationships. Just as we have relationships with books, so we have them with people.

Mary is a treasure to John - John likes (or loves!) Mary very much, has warm feelings for her, values her company/friendship (because she is so nice)

The "to" implies that John has said this is so, that these are his avowed feelings being described. She is "everything" to him.

The Bible/The Lord Of The Rings/Whatever is a treasure to John (because he likes it and finds it an agreeable/inspiring/cheering companion)

Mary is a treasure for John - John values Mary (because she helps him a lot with his work, cleans his house well, gives him good advice)

(The "for" implies that this is a judgement of other people)

(In colloquial UK English, a lady who comes daily or at intervals to clean your house, and does it reliably and well, so that you are glad you found her is sometimes called a "treasure")

The Encyclopaedia Brittanica/Oxford English Dictionary is a treasure for students (because it is so full of information)

Of course "treasures" are not only valuable, they are only rarely found.

Many thanks, Contrex, for the detailed, analytical and clear reply.

Best wishes.