This work is all about creating new races of intelligent (more or less) creatures for your game. The cover is virtually obscured by 'Compatible with' logos ranging from fantasy to science fiction systems, but never fear if your chosen ruleset isn't there, it is generic enough to be used whatever game system you prefer although those are the ones explicitly supported.
There are two ways to use the book. Either grab a handful of dice and start rolling, see what you come up with, or use the sections as guidelines to drive your thoughts, ensuring that you consider all the aspects that will be useful in creating a species that you'll be able to bring to life in your chosen setting. Or, of course, a combination of both: maybe starting by rolling dice then change the things that don't suit the concept you're forming, or only roll when you don't have a clear idea about a particular aspect. However, bear in mind that it can be fun working out how an oddball species survives and thrives - the randomly-designed ones can be the most interesting! (Bipedal naked mammals, who'd have thunk THEM up?)
The first table covers general species biology while the second one deals with humanoid species biology. The idea here is that the first table can generate anything from animals to sentient races, whilst the second is aimed at creating the sort of 'aliens' that you find in science fiction films, the ones that when all is said and done are human beings with makeup and prosthetic effects on.
Then if you are after a strange-looking race (or rolled for something 'deviant' on whichever of the first two tables you used) there are some weird things you can mix and match into the race you are building. This is followed by a set of notes about different sorts of organisms, which may help you expand on the brief description that you have derived so far for your new species. They are quite broad, and contain several inaccuracies - mostly deliberate ones which are covered in the notes, like lumping dinosaurs in with reptiles. A couple of minor quibbles include the statement that a sense of smell is not much use if you're aquatic - try telling a shark that!
The notes also cover habitat and structure (of the body, that is). Then the discussion moves on to the core of the matter: creating new PC races for your game. This is handled by offering 'kits' for several game systems (13th Age, Dungeon World, Fate Core, Legend, Pathfinder and Traveller) which you use to derive the system-specific information that you need for the race you have just invented. As demonstration, each kit is used on the same basic race, a Turtle-Man from Chelonian Press's Turtle Lords RPG (1983). These kits come with explanatory notes showing you how to make the conversion from the core information generated here into the game system of your choice.
And then there's another set of tables, these ones are to enable you to devise the sort of society in which your new species likes to live. Everything from government types to their values and attitudes, as well as strengths and weaknesses, nature, assets... as well as further tables on the civilisation's past and current situation. Not only does this give you a good overview of what a society is like, it also feeds into the 'Societal Conflict System' that is presented next. This isn't so much about actual warfare as about determining the underlying geopolitical situation. Look at the news. In some part of the world, people are actually fighting each other, in others the diplomatic situation is a bit tense and in some places there is more or less a state of peace... but there are always tensions, allies and nations which do not get along and the like. This is about generating this kind of background situation which may or may not actually impact on your plot, but is part and parcel of the setting in which your game is being played.
Naturally character actions may have an influence: they are the stars of the show! This may be direct - perhaps they are soldiers, spies or diplomats - or incidental... who knows, maybe they created an international (interstellar?) incident without even intending to... and the intention is that the GM should use this system in between game sessions to model what is going on in the rest of the world while the party is off doing its thing. It's a neat concept which should add interesting depth to your game.