Grass by Sheri S. Tepper

Generations ago, humans fled to the cosmic anomaly known as Grass. But before humanity arrived, another species had already claimed Grass for its own. It too had developed a culture......

Now a deadly plague is spreading across the stars, leaving no planet untouched, save for Grass. But the secret of the planet's immunity hides a truth so shattering it could mean the end of life itself.

Pros: several complex plot lines that all get resolved satisfactorily, interesting characters that develop over the course of the book, detailed world building - for the planet Grass as well as Earth and the rest of the universe (even though the rest of the universe isn't mentioned much)

Cons: can't think of any

Grass is a planet with no reports of plague victims in a universe of worlds dying of the plague.

Lady Marjorie Westriding Yarier and her family are sent by Sanctity, the dominant religion in the universe (though they are old catholics), to see if it really is free of plague, and/or if there is a cure for the plague on the planet. They are chosen because the nobles on Grass ride the hunt, and the Yarier family is good with horses.

Unknown to them, the bons ride Hippae, and the Hippae are not horses. They are malevolent creatures with unknown motivations.

The green brothers live on Grass, digging up the ruins of a civilization that died out centuries before. A race that may have died of the plague.

There are a lot of politics and a lot of revelations you won't be prepared for. It's a creepy novel at some parts, a tragic one at others. It is well worth the read.