Two realtively unknown books that should be mandatory sci-fi reading:
1. Armor -- John Steakley. This book is in the running for my #1 favorite sci-fi novel of all time.
2. The Dragon Never Sleeps -- Glen Cook. Out of print and really hard to find. If you do find it make sure you buy it. Ebay is a good place to look.
Lastly my fav sci-fi reference:
The Internet Top 100 SF/Fantasy List