You shouldn't assume that just because other historians don't like a given book that it's part of some kind of political conspiracy. That's pretty silly, very anti-intellectual, and reflects a deep ignorance on how scholarship works. We don't read one book and say, "welp, that's it, I'm convinced!" — we read many books, we look at the primary source literature ourselves, we look at the big picture as well as the small details. If we don't like something, there's probably a good reason. Want to know that reason? Read reviews, read other books!

In the case of Giangreco, his work has value but it is extremely one-sided and leaves out a LOT. Giangreco is a hardline defender of the "orthodox" interpretation of the use of the atomic bombs, and there are real, serious problems with that, if you study it seriously.

For example: Giangreco is obsessed with hypothetical casualty estimates from an invasion, and claims they had a big effect on Truman's "choice" to use the bomb. This is the lynchpin of his thoughts on the atomic bomb question. But it's pretty clear if you look more carefully at the history that 1) the only casualty estimates that were seriously discussed at the time by the military heads with Truman with relatively low (compared to Giangreco's huge numbers), 2) they were a factor in thinking about the end of the war but hardly the decisive one, and 3) Truman himself never did any kind of moral "calculus" where using the bombs was part of the avoidance of invasion — the plan was to use the bombs and invade. Giangreco's whole argument ultimately rests on hypotheticals that don't reflect what the historical actors were really concerned with at the time, in an effort to justify the bombings. You're going to tell me that that's the non-political, unbiased argument, and everyone else is some kind of leftist activist who can't bear the truth? You can see why this is insulting to practicing historians.

It also doesn't help that Giangreco is pretty nasty when it comes to dealing with his critics. Does he engage them in the spirit of scholarship and debate? Not at all — he calls them "vampires" and "crackpots." Giangreco's critics have done a far better job of engaging with Giangreco than he has done with engaging with them. That matters for scholars; when you start descending into ad hominem it's an indication that your argument is not very good.

If you want a very level overview of the atomic bomb interpretations, and where Giangreco sits in that scheme of things (and the problems with his work), read J. Samuel Walker, "Recent Literature on Truman’s Atomic Bomb Decision: A Search for Middle Ground," Diplomatic History 22, no. 2 (April 2005), 311-334. Walker is extremely even-handed, is a very serious historian himself, and talks about the deficiencies of both the hardline orthodox approaches and the hardline revisionist approaches. Most historians today have incorporated the "correct" aspects of each of these arguments into a synthesis argument that does not make nearly as large claims as either of them.