Neighbors like that are the worst and right? I once told mine, 'Borrowing is like taxes - inevitable until you say no.' For the parking spot, try a friendly 'Hey, that's my sacred ground, find your own turf!' Keeps it light but gets the point across without starting World War III.
Keep books out of direct sun and heat by shielding shelves and keeping the room cool with steady humidity. Avoid plastic and tight stacking and allow airflow, turn page edges away from the light, and rotate the display so spines take the fade.
