Reading autobiographies is usually not me thing. I stumbled upon Dear Theo when I was looking for books about innovative thinking. I was not disappointed on that respect.
But in addition you get a good feeling about what life meant back in 1870-1890. The book purely consists of letters that Vincent wrote to his brother. So, this is not some historian that describes a time he might not even have lived in, but the real thoughts of a person from that time. And I can tell you, Vincent was not some dumb-ass. It amazes me how people 140 years back thought pretty much the same thoughts as we do nowadays; 40 years before the first world war, right after the industrial revolution.
And, of course, there are tons of insights about Vincent's pictures, his techniques, how his painting changed over time, interactions with painters of renown, books he read and liked and whatnot.
I myself am not an art-guy. But if you listen to what Vincent thought about his daily painting and how much efforts and experimenting he put into this, even I listened with great interest.