"The philosopher is not free to dispense with Wagner."
-Friedrich Nietzsche, The Case of Wagner
When the book was good, I thought it was very good, but it wasn't at all times. It's more encyclopedic than analytic, at times even gossipy. Did you know that Virginia Woolf went to a benefit costume ball as a Valkyrie in 1909? Me neither. Not sure that tells me much about Woolf, though it might say something about how ubiquitous Wagner was.
Years ago I read Ross' The Rest Is Noise, his history of twentieth century classical music, around when it came out in paperback. I really liked it, even though most of it went over my head. (And I'm pretty tall.) I don't know that much about serious music. A diminished seventh, you say--is that music, baseball, or planetary science? I wouldn't know one if it bit me on the ear. Still, sometimes I like a book that's too hard for me, it leaves me wanting to know more.
Because Wagnerism addresses Wagner's presence in literature and film--and politics--more than in music, this book worked differently for me.
Ross' organization is a mix of chronological and by topic. The first figure Ross covers is Nietzsche, an early Wagner disciple, at least until he wasn't, and the most prominent. Ross gives himself 60 or so pages on Nietzsche, and is lucid and helpful. Nietzsche and Wagner is a topic on which books could be written--and have, with more than one of them by Nietzsche himself. Subsequent chapters are on French Wagnerians (Baudelaire was an early booster), British, American, Austrian, Russian, black (W.E.B. DuBois was a fan), gay, Jewish, feminist. Right-wing, but also left-wing. Well, everyone actually was listening to Wagner for a while.
When Ross gives himself space he's at his best. This includes his writing about Nietzsche and Baudelaire. There was interesting stuff on Joyce. Wagner is important to Thomas Mann, (less so to Heinrich) and Thomas Mann's attitude toward Wagner changes over the years. I was particularly interested in Ross' take on Mann's Joseph saga, which he reads as a direct challenge to Wagner's Ring Cycle: both tetralogies, both investigations of myth, but Mann dealt with Old Testament--that is, Jewish--subjects, which Wagner himself wouldn't touch. There was some good stuff there.
On the other hand Ross speculates that Wagner is behind Adrian Leverkühn, the hero of Mann's Doctor Faustus. But how can that be when *everyone* knows that Leverkühn is Nietzsche + Schönberg? 😉 There needed to be either more or less to that argument.
Ross is at his tip-top best on Willa Cather. He devotes an entire chapter to her, largely on The Song of the Lark, and she shows up in some of those other categories listed above as well. Very well worth reading if you care about Cather.
Wagner in World War II is the emotional heart of the book. Ross is at some pains to remind us that lots of people who did not like Naziism, were never going to like it, still liked Wagner.
Wagner's politics were complicated and probably not well-thought-out. He got himself in serious trouble supporting the anti-monarchical revolutions of 1848 and it took him years to get out of that trouble. He was pretty seriously pacifist. He meant his anti-Semitism though, publishing a vile article first anonymously, and later doubling down by re-publishing it under his own name. One of main reasons Nietzsche, not exactly known for his compassion, broke with Wagner was his abhorrence of Wagner's anti-Semitism.
Hitler's Wagner--and Hitler really did love Wagner--was not all of Wagner. But how big a chunk of the total Wagner was Hitler's Wagner? The jury's still out on that, even I would say, in Ross' mind.
"Perhaps [George Bernard] Shaw hung back from direct engagement with Wagner because he wished to avoid placing himself in competition with the Meister." 
Is this that same Shaw who cheerfully bashed on Shakespeare?
"Cy Twombly listened intermittently to Wagner while working on his ten-painting cycle Fifty Days at Iliam..." 
I listen to Wagner intermittently, too, though in my case, the inters are pretty danged mittent. Are my posts therefore Wagnerian? Well, maybe they go on too long...
Ross ends with a brief history of his own listening to Wagner. He was not initially a fan, it seems, but then in his twenties got excited about Wagner. But wondered, should he?
Anyway, good when it was good, and very good when it was very good, as we say in the Department of Tautology. If anything in the subject interests you, it's quite readable and often astute. It didn't blow me away like his first book, though.
It's one of those pan-European books that might do for a lot of countries, but I'll stick to the basics and count it for Germany: