This index only includes “treasures” from two collections - there’s a more comprehensive index here: https://www.maa.org/press/periodicals/convergence/index-to-m...

My personal favorite is this one, the only mathematical proof ever published by a U.S. President — James A. Garfield’s 1876 proof of the Pythagorean Theorem: https://www.maa.org/press/periodicals/convergence/mathematic...

Oxford's Bodleian library also recently (partially) opened its post-1500s books and manuscripts collection online:

https://archives.bodleian.ox.ac.uk

(Doesn't include their famous illuminated medieval manuscripts collection though.)