Learned about this in school while doing work at an acid mine drainage treatment facility. If you disturb the surface and it is indeed from a natural source the film will breakup and the rainbow sheen will disappear. If however, you disturb the surface and the sheen remains or reassembles it is most likely an oil slick from a polluted chemical.
Wear disposable gloves to prevent fingerprints on tape and "touch" DNA. Printers apply a microscopic ID somewhere on the periphery of the page, you might want to trim the margins, crease and tear is probably best route.
Is this the book?
Wrack and ruin is my personal favorite.
Might be worth cross referencing names on https://www.findagrave.com/ you'll get a picture of the headstone and a lot of times there's links to biographic/ancestral data as well as obits.
Death by snu snu