This short work is jam-packed with thought-provoking ideas for how to use different racial mixes in your game. In the Introduction, the author describes how he's always enjoyed exploring new races in his own playing, as well as reading about exotic monsters and wondering how they'd work as characters. Here, then, are some tools and ideas if you'd like to do the same.
Chapter 1: Half-Breeds in Your Game looks at the ups and downs of making much use of them, and at some of the things you need to think about when deciding what you're going to allow. The liklihood is that they will be less common than racially-pure individuals - but why should this be, and what effect will a few of them have on society? There's always good role-playing potential in being a bit different, and that to my mind is the real advantage of having half-breeds. It is not as disruptive as introducing a complete new race, and you can degree that due to social or even biological reasons they are quite rare.
The main part of the book is Chapter 2: Half-Breeds, in which no less than 12 new possibilities are presented, complete with ready-made backgrounds and all the rule mechanics needed to play or referee them. There are some fascinating concepts here, quite unlikely half-breeds but ones for whom a comprehensive rationale as to why they exist is given, one which in many cases could give rise to a whole series of adventures. I think I fancy an ophidian - result of a medusa seducing some powerful and influential human - but whatever your tastes, if you are of a mind to experiment with more than the standard fantasy races there is plenty of scope here for you and your DM to find some interesting characters and things to do with them.
Next, Chapter 3: The Demiurge presents a way in which those DMs concerned about internal consistency in their alternate reality can explain the most unlikely of half-breeds. The demiurge are mysterious beings which come from outer space and indulge in abduction of and experimentation on the sentient beings they encounter... with surprising results, at times. They could make a good antagonist, whether as a one-off or as the focus of a whole UFO-based campaign.
Finally, Chapter 4: New Monsters gives details of the giant cockroach and the gorilla, which at the time of writing have not yet made their appearance in Dungeons & Dragons 4e, but which are part of the mix that forms a couple of the half-breeds in Chapter 2.
A simple concept that goes straight to the heart of the 'what-if' mentality that drives much of fantasy, the ideas herein - and those that just thinking about them will spawn in your own mind - can empower an added dimension to a fantasy game, with as much social and personal interaction as you and your players might wish for. Many of the half-breeds are good in a brawl or other encounter as well, so however you like to play your game there may well be something of use here.
Castoffs and Crossbreeds