Here's a cheap and fun low-tech way to understand values.
Make a value scale using pencils using cardstock or Bristol paper. Slice a window above your scale.
Use any B/W photo and compare the values you see with your scale.
See below.
1. Value scale 2B pencil on cardstock.
2. Choose a black and white image.
3. Analyze the values seen in the image.
4. You can do some fun things with this tool, such as pick out three value levels, and try to do a drawing with only those....You can also hold this up to color photos or drawings and by squinting, see the values in color pieces. It really helps me to do this when I work in color, since I often have too many mid-tone values.