It stands alone. I read it first before Peace War and didn't find Peace War as good, though still very enjoyable.

I believe Marooned is a stand-alone story.

Fire Upon the Deep is the first of 2 (or 3?). I highly recommend that. Older readers will love the Usenet jokes.

It builds on the Peace War, and I'd read that first. (But also, both books are fairly short, much smaller than Fire Upon the Deep.)