Actually, House of Zeor isn't the first book chronologically. It was the first book published, however, and is probably the first book a newcomer to the series should read. The Sime~Gen universe is complex and can be a bit confusing; House of Zeor is a relatively simple but compelling introduction to it. Hugh Valleroy, a Gen who, like all Gens, hates and fears Simes, must go undercover into Sime territory to rescue the woman he loves. He meets Klyd Farris, a different kind of Sime. A Sime who doesn't kill. The book is a classic "getting to know an alien culture" story, and the reader comes to understand Simes as Hugh does. It's a fast read, full of adventure and intense male bonding. If you liked Trek Classic or early Cherryh, you'll love this book.
House of Zeor was first printed in hardcover back in the '70s, and has been reprinted in paperback since then. It's out of print as a standalone book now, but you can often find used copies. It's currently available as part of the Unity Trilogy, along with two other Sime~Gen novels, Ambrov Keon and Zelerod's Doom.
The Official Sime~Gen Web Site - Cluttered and hard to navigate, but persevere, and there's fanfic, mailing lists, news of upcoming books, and other cool stuff.
The Secret Pens - The unofficial fan site, where the good stuff is ;-)