The importance of thorough inspection when there is any suspicion of infestation
This book arrived wet. After freeze-drying it was inspected. The paste-downs showed some brown staining of the type that commonly bleeds from binder's board (note "tide lines" at the bottom). The line of discoloration (white arrows) looked like it was probably color that had bled from loose papers that were in the book at the time it got wet. As there was only discoloration and no outward sign of mold, it didn't present cause to suspect fungal activity.
However, the book did show minuscule traces of mold growth here and there in the gutters, a sign of possible infestation in the hollow of the spine. The text block was disbound from the case. Sure enough, there was slight mold growth at the board edges (red arrows). At least it appeared to be slight.
On further inspection, this was found to be the situation. The boards had delaminated in response to being wet, and mold was flourishing within the microenvironment it had found inside the boards.
As the text block suffered no objectionable staining and responded well to flattening in a press, it was thoroughly cleaned and the spine washed with alcohol/water mixture, followed by drying outside in direct sunlight. After binding into a new case it was kept and observed for about a month, at the end of which time it appeared to be fine.
So, it turned out that our seemingly innocuous stain (again, white arrows) was in fact entirely the result of fungal activity; it just offered an unusual view of it. But otherwise, this book showed barely minimal visible signs of a mold problem. If it had not been thoroughly inspected it may well have been returned to its collection in this condition. Given that the book is from a circulating collection, the potential for distribution is an item of concern. Not only would it pose a risk to its neighborhood in the stacks, but a patron's book bag, car, home, and classroom(s) would all present opportunities for new infestations seeded by a contaminated book.