"Portrait of an Unknown Woman" by Vanora Bennett. Sir Thomas More and painter Hans Holbein, through the eyes of a young woman in the household. If you're interested in historical fiction.

I recommend "How to Survive Modern Art" by Susie Hodge. It only covers the past century (if you couldn't guess from the title) but that's ok as there was a plethora of movements in the 20th century compared with previous ones. It's beautifully designed with lots of pictures and takes a page or two to describe different movements and provide examples. She is very engaging and non-pretentious and suggests what to look for in different works and why they are important. She also keeps each entry fairly short. I got this from the library and liked it so much I bought my own.

I haven't read yet, but heard great things about, The Lost Painting: The Quest for a Caravaggio Masterpiece by Jonathan Harr.