What you need, in the least, is a psychrometer. You can get these 'gizmos' at the mall in some specialty shops. Science shops and Home Centers, so I've seen, carry a 3-in-1 type of unit. It measures temperature, humidity, and barometric pressure. This is probably what you want.
I do want to suggest that you speak to the local librarian about this, or consult a book on the subject. I'm not trying to read too much into your post but if you have valuable books, BOTH temperature and humidity need to be monitored; not just humidity. I know the absence of moisture in the air is not healthy as is excessive moisture in the air is not healthy. In a library, a fine balance is maintained.
Happy trails and hope this helps. Here's a link that would give you a start. Scientific Sales. Follow the Consumer Products URL to the Temperature & Humidity URL and go from there.