I'm not going to read a book by some lying neoconfederate wacko if that's what the author of that book is. If Confederate POWs were massacred, you should be able to link me to historical proof.
Also, on January 24, 1864, long before the bulk of men died at Andersonville, Judge Robert Ould, Confederate Agent of Prisoner Exchange, sent the following letter: The US Secretary of War turned this offer down.
Why was it turned down? It makes no difference on the Confederate side regardless. Failure to release those they couldn't feed was murder and holding them hostage to demands was terrorism.
In my opinion, the Secretary has taken and obstinately held a position of cold-blooded policy, (that is, he thinks it policy) in this matter, more cruel than anything done by the secessionists. ... In my opinion, the anguish and death of these ten to fifteen thousand American young men, with all the added and incalculable sorrow, long drawn out, amid families at home, rests mainly on the heads of members of our own Government..."
By the way, the Immortal Captives book is extremely well documented. I bought my copy at Andersonville Prison in the Park Service store.