TempPro TP50 Digital Hygrometer Indoor Thermometer Room Thermometer and Humidity Gauge with Temperature Humidity Monitor(Previously ThermoPro)

This looks like a humidity issue. Is it hot/wet/humid where you are? You might consider running a dehumidifier in the space where your books are stored. Somewhere between 35–50% RH (relative humidity) is generally considered appropriate for book collections... most digital thermometers will also display RH.