I enjoyed Varley's Red Thunder, to which this is a sequel, but people to whom I recommended it didn't seem to find it as much fun as I did. Let that be a warning to you.
Like Red Thunder, Red Lightning harks back to Robert A. Heinlein's SF juveniles of happy memory, but with a lot more sex, drugs, and cussing then RAH could put into his books. Come to think of it, there's more of those things than in Red Thunder, which takes place a generation after the earlier book.
An object strikes the Atlantic Ocean and causes a horrific tsunami that wipes out millions. Ray Garcia-Strickland, son of Manny, the guy who made the first Mars flight in the previous book, is the narrator. He and his family leave Mars and go to Florida to look for Ray's grandmother. Things are terrible, and about half the book is set there. The U. S. is far down the road to totalitarianism, and things become even worse when Mars is invaded (three times) in the second half of the novel. You'll either think it couldn't happen here, or you'll believe it could.
Uncle Jubal is a major player, as he was in the first novel. His brain works in wondrous ways, and his latest invention is . . . I'm not telling. If you read the first book, you'll remember the "Squeezer," another major player in this one, and his new invention almost tops it.
I'm no expert, but I see Heinlein's fingerprints all over this book. For me, that's not a bad thing. But before you rush out and buy it, you might want to re-read that first paragraph up above.