This product contains a comprehensive review of what breath weapons actually are, how they work in terms of game mechanics and even takes a look at how they might work in terms of the alternate reality of your game... what is actually going on inside that creature just before it breathes its 'whatever' at you. This last is of particular interest to anyone who likes to understand the 'ecology' of the monsters that they use in their games.
At places, particularly in the first few pages, the text reads more like notes jotted down as the author thinks about breath weapons rather than a finished product. By the time we reach the 'in character' descriptions of breath weapons it has settled down to a more polished delivery.
An interesting feature of the 'in character' understanding of how breath weapons work is that it has been written at the level of knowledge appropriate to a quasi-mediaeval fantasy world. So, for example, an electrical attack is not really understood - as a 21st century person with life-sciences training, I'd be thinking in terms of the way in which a creature like the electric eel generates its charge, but here we have thoughts of it being akin to an electrical storm and ending with 'The alchemists are working on an answer to that'!
Then we move on to the process of designing a new breath weapon from the ground up, including calculation of CRs and creating a template for creatures that will use it. This is followed by several new breath weapons that have already been designed for you to use. Many of these are based on spell effects like Fear or Move Earth.
Finally there are some 'breath weapon related feats. These enable the creature wielding the breath weapon to modify or otherwise control it in a similar fashion to a magic-user utilising metamagic feats to alter his spells. At the very end there's an appendix that looks at areas of effect (with neat charts) and ways of calculating the course of a jagged, curved or chaotic breath weapon.
There's a lot of useful stuff here about the art and science of breath weapons, and with a modicum of editing and expansion could be a contender for 'Compleat Book of Breath Weapons'... it just gives out an air of having been a little rushed in the writing, perhaps a deadline caught the author unawares.