An interesting idea. The "Behind the Music" or "Ecology of" a particular spell. It gives you some fluff and background material on how the spell was created and used and even has a couple of newer spells. This will not be everyone's idea of a good supplement, but certainly helps keep the magic fresh and interesting in the game. Plus it is a great idea for researching new spells or even finding "lost" spells.
Every spell has a story, this one costs you a buck-50.