I source my materials from library sales, thrift stores and yard sales and I don't buy or accept a book unless it's damaged or otherwise not likely to be sought after because of its condition or because the information is out of date. On the contrary to feeling guilty, I am excited to turn the discarded books into lasting works of art.
Book paper quality is usually a bit better, depending on how old the book is; the paper is thicker and different printing techniques are used, ensuring the longevity of the pieces, with more vibrant colors. I like the relative thickness of the paper and the texture the layers of paper give my work.

A frame adds so much to the value of the piece, it's important to do it right. It's essential that the surface of the collage not be in contact with the glass, so at the very least a mat is required. Ideally, a custom frame with a spacer keeps everything safe.
Please don't hang unframed or even framed pieces in areas like bathrooms, or over a kitchen sink where it's likely to be exposed to excessive moisture. My collages are sealed with a gel medium and an acrylic spray but at the end of the day, they are paper.