Books with foxing usually do NOT need to be sent to Preservation for that reason alone. We have mentioned foxing here -- found in some older books -- because we sometimes get questions about this condition.
These brown-red stains are circular or irregular in shape. The origins of the marks are not yet fully understood (e.g. fungi? oxidation?) but they do sometimes occur. They tend to be distributed more evenly over the page, whereas mold tends to form on either the outer edges of the page or near the spine.