The Essential Guide to Alien Species is a reference book released by Del Rey in 2001, written by Ann Margaret Lewis, and illustrated by R. K. Post. It covers 140 species in detail and an additional 25 in a section in the back of the book. The book comes with a handy chart with the planets and their constituent species, a pronunciation guide, and a timeline detailing events from 3,000,000 BBY to 30 ABY. Like The New Essential Chronology and Galactic Phrase Book & Travel Guide, this book "exists" also as an in-universe publication, written by Mammon Hoole.