I'm working on a book about famous works of art. For example I want to give an in-depth analysis of the Mona Lisa and use a picture of it in my book.
Can I do this and still sell the book?
Items which are in the public domain are free for use in any way. This includes works of art such as the Mona Lisa.
You can get far more information about this at: http://fairuse.stanford.edu/overview/public-domain/ Here's a quote from that site:
If a book, song, movie, or artwork is in the public domain, then it is not protected by intellectual property laws (copyright, trademark, or patent laws)—which means it’s free for you to use without permission.
Photo of Public Domain Work
However, if you have a photo of the Mona Lisa that someone else has taken then they own the rights to the photo and that may be protected. However, many times you can obtain an image of the public domain work from wikipedia and the rights to use the photo from their may be an open license.