Do you have a pressed flower in your book? A letter? It's "ephemera"

 The Ephemera Society of America shares a long webpage on the term's definition with pictures and text covering a wide variety of ephemera.

Precious primary source information . . . that is what the ephemera world considers its bits and bobs of vintage (and current), usually paper items. Much of it was likely expected, back in its day, to be briefly useful then discarded. Today s auch items which have survived the vagaries of time often reveal things we might not otherwise ever learn.

 (https://www.ephemerasociety.org/definition/)