Is it obligatory to change the clothes a person was wearing when sleeping on dry, impure pillows covered with a pure cover, out of fear that the cover might be uncovered during sleep?

The dryness of an impurity does not purify it. Placing books and papers containing the remembrance of Allah next to something impure is not forbidden, as long as they do not touch the impurity. However, it is better to keep them away from impure things as an act of veneration for the rituals of Allah. If the pillow has a clean cover, there is no harm in placing books next to it or on top of it. Impure pillows are purified by washing them until the impurity is removed.