I received a book today that has brown spots/stains all along the page edges. The spots don't extend to the text but are basically scattered on the edges of almost the entire book.  I'm not sure if this is just dirt or mold. The book is readable but I'm almost afraid to touch it without knowing what caused the spots, especially if it's mold. I tried to clean the spots but they don't come off.   What does mold on a book actually look like and how can you tell the difference between mold and just plain dirt.

Well, is it an older book? Books that are aging often develop yellow-to-yellowish-brown spots called foxing that can be on the page edges, on the covers, or developing on the pages, too. It is caused by a chemical reaction of the paper over time, and is not harmful. It is postable here on PBS.

Mold usually smells like mold, and in my experience, its usually black-ish spots or white-ish spots.

Dirt usually looks like dirt - usually grey-ish brown, with no smell.

Very scientific, I know. Hope this helps!

Neither mold or dirt (that doesn't come off = soiled) are allowed, but I suspect it is foxing like Sara mentions. Perfectly harmless. If you search the forums for "foxing" you should find some good pictures of it.