histograms are easy - 1 through 255 in tonal range, 0 is totally white, its the solid line on the left, 256 is totally black and is the solid line on the right of the histogram. Keeping the exposure within the tonal range of 1 and 255 is key to getting a proper exposure. Some newer DSLRS will display not only the main histogram, but the histogram for each color channel i.e. red, green, and blue.

There is not much difference between film and digital as far as exposures go, so a book on film photography doubles as a book on digital. However, I don't think any book can do as much as the actual shooting and practice goes. If you want to read up on digital photography - read up on the printing and photoshop side of things - that I think would be a lot more practical and necessary than a book on the cameras.