Mistral! I'm shocked! How could you not suggest Space Viking (doesn't get more opera than this) and Cosmic Computer aka Junkyard Planet? Both by Piper. Space Viking has even seen a reprint in the last few years, picked up a new copy a couple months ago.
The Berserker books by Fred Saberhagen (and now other authors)
Still waiting for someone to write a Berserker VS Bolo book. That would rock!
I figured if he liked the 4 I suggested by Piper he'd find those on his own. I didn't suggest A Planet For Texans
either but its one of the better stand-alone novels he wrote. For that matter, there's Earthblood
by Keith Laumer-now that's opera at its finest.