This supplement is specific to the Traveller universe, but the data it contains could be easily adapted to any interstellar milieu. Since Traveller tends to be human-centric, it's not really essential to a Traveller campaign, although it's a decent supplement worth considering.
The book contains data on 16 intelligent races. The entries vary somewhat in content, but most entries provide information on the species' history, physiology, psychology, society, equipment, and a template for creating a character of the species in question (some but not all entries provide maps and data on homeworlds, government, language, trade practices, sample NPCs, etc).
The entries are sufficiently detailed to allow you to play a character, but at less than 10 pages per entry, there's not a lot of room to go into real depth. My biggest criticism would be that the alien races don't seem particularly alien... lizard people, octopus people, rhino people... almost all of them seem to be intelligent versions of terrestrial animals, albeit perhaps with a slight twist. Not much that struck me as truly alien.
The species detailed are:
Ael Yael (pterodactyls that hate megacorps)
Ahetaowa (psionic plants)
Bwaps (aka, Newts; lizard beaurcrats)
Evantha (flying lizards)
Girug'kagh (translators for the Centaurs)
Githiaskio (squid people)
Hhkar (dinosaur people)
Hlanssai (cat people)
J'aadje (they look kind of like ET to me, lol)
Jgd-Ll-Jagd (gas giant dwellers... one of the few really alien-seeming races)
J'sia (giant worms)
Schalli (dolphin people)
Shreikers (spider people)
Tezcat (they look a lot like goblins)
Valkyrie (tapeworms who control a host creature)
Virushi (rhinoceros people)
The bottom line: not a bad book, and worth having if you want to add more alien flavor to your campaign.