This is a thought-provoking resource aimed at helping the most inventive world-builders design their own sentient lifeforms with which to populate their universe. Quite a tall order, especially in under 50 pages!
It all depends, of course, on your universe. If you tend to the concept put forward by EE 'Doc' Smith in his Lensman series that the majority of life in the universe was 'seeded' by one ancient race (Arisians, in his case) they are all going to be similar and characters in it are likely to be of this dominant type. Yet even there, there were several varieties, not everyone was 'human' in appearance although the majority were. Others hold that virtually any kind of lifeform, given the opportunity, might develop to the extent of becoming sentient - if that appeals, try James White's Sector General series: he is equally brilliant at dreaming up exotic races and finding things to go wrong with them!
Here you have the tools for designing sentient races from the bottom up, beginning with deciding what sort of lifeform they developed from, then adding in a range of options from numbers of limbs and digits to psychological characteristics. A word of warning, even if you just grab a handful of dice and go straight through the tables rolling furiously, creature creation is not a speedy process - you won't be doing this mid-game to populate the generation ship your characters have just encountered!
What you do get, though, are lots of thought-provoking suggestions to chew over. You may need to remind yourself to stop designing your sentient race and write an adventure.
If you find the process intriging, delve into some of the extremes of earth biology and the speculations rife amongst such as NASA astrobiologists, watch the Discovery Channel or equivalent.. and don't forget science fiction. Plenty of authors are good at coming up with unusual races and societies, and you can mine them for inspiration... but this work is a good starting point and shows you clearly what you will need to consider!