Often at such exhibitions you see sketches drawn before a famous work was made. In his case many of these drawings were done afterwards - sometimes to send them to his brother or friend. Occasionally they were monochrome with little notes specifying the colors he used. Not bad getting a letter with an original Van Gogh in it.
I spent some time leafing through the book. He was a self taught artist who bought lots of books about how to draw or paint and did the lessons in them; he continued this through his whole short life.
This exhibit closes on Dec 31. We got there at about noon; the exhibit was crowded (this was a Saturday) but the queue to get in was just a handful of people. When we exited an hour and a half later the line was extremely long.
There is also a Prague (1347-1437) exhibit there that we saw - it closes Jan 3. I'm moving Prague up in priority in my list of places to visit.