Fire is, it's probably fair to say, the most widely used of the four elements when it comes to spell-casting (at least by adventurer types). Despite this, the Genius Guide to Fire Magic does a good job of adding yet more options to those already available as standard.
There are 14 spells in the booklet, which is less than in the equivalent volumes on air and earth magic, but they are all clearly tied to the element, while still managing to be original and different to those in the standard books.
The focus is primarily on spells for druids and sorcerer/wizards, but all spell-casting classes are covered. The spells range from 1st to 7th level, and most deal damage in some way - although there are also some spells that illuminate in various ways. The use of smoke, steam, and molten glass within spells allows for more variety than one might expect if you simply stuck to flame.
Unlike the earth and air volumes, there are no new cleric domains, sorcery bloodlines or the like here - there isn't much need. However, there are similar class options for inquisitors, oracles, and witches, a template for fiery monsters, and two new metamagic feats for altering fire spells.
It's perhaps slightly less use than the earth and air volumes in that it's shorter, has fewer spells, and class options only for non-standard classes. But, taking that into account, and considering how much coverage fire already has, it's an impressive job of expanding something that already looked to have the bases covered.
As an aside, since this came out a while ago, one wonders why SGG never got around to a volume on Water magic. Ice, sure, but not water. Pity.
[5 of 5 Stars!]