The opening discussion talks about how most spellcasters, arcane and divine alike, are liable to have at least dabbled with magic involving either light or darkness, or indeed both; but that for those prepared to delve deeper great power is to be found. Most spells with either the light or the dark descriptor are to be found in the Evocation school of magic, and can negate or dispel those with the other descriptor.
There then follows a collection of spells around the themes of manipulating light and darkness, drawn from a variety of sources with some being original and other having been modified from the original source (or in the case of those from the core rulebooks, the errata have been applied).
Overall, it's an interesting thematic collection, and could be of use to either someone playing a spellcaster who'd like to specialise in this area or the DM preparing spellbooks for his NPCs (or to be found as look); a timesaver if you don't want to delve through all your spell collections. But it lacks much in the way of commentary to link the spells within the light/dark theme or show the particular ways in which a specialist in light (or darkness) might use them to greater effect than a mere dabbler. A good start, though, but it could do with more work to become the sort of scholarly study mages really want!