Traveller (Mongoose 2nd Edition) - Aliens of Charted Space: Volume 1

Portraying an alien race or a member of one is a challenge. It is easy to pick a few significant characteristics and overplay them, creating a caricature, or turn them into a generic human with a different hat by not accentuating their differentness. The key to a plausible alien is to understand their motivations and what makes their society work. Nobody does anything without a reason, even if the reason is unknown or incomprehensible to outsiders.