Much worth reading; I'm no expert on the substance, but it's certainly a fun and interesting read, and my sense is that the criticisms of Douglas, from what I've heard, are indeed quite apt. (Certainly Douglas's constitutional work, with which I am familiar, betrays the flaws that Posner identifies.) Thanks to Orin for the pointer.

Scot Powe, a constitutional law professor I much respect, who knows a good deal about Justice Douglas -- he clerked for him, and he wrote a history of the Warren Court -- writes:

The problem is that Posner believed everything Murphy wrote when in fact much was in error (as my review in Reviews in American History showed).

Here are some more specific criticisms of Murphy's book, from Powe's review, which is also a good and short read (paragraph break added):

Murphy believes [Douglas's claim of having been striken with polio as a child] was a fabrication. Yet most of Douglas's symptoms and treatment were consistent with polio. We will never know what Douglas had, but Murphy has not shown what he claims, that Douglas knew he did not have it.

Similarly, Douglas was always a top student and all biographies note he was number two in his Columbia Law School class, but this is not quite accurate. He was near the top of his Columbia class, but not number two. Again, however, Murphy assumes Douglas is lying, although his evidence does not show that Douglas did not simply make an honest error. [endnote: The records of Columbia's class of 1925 are still not public.] Furthermore Douglas had been terribly sick at two, and doctors thought he would die; he worked all kinds of jobs for the money that his family always needed; he was a whiz at school and never could devote himself full-time to his studies. And not many people would survive, as he did in 1949, a horse rolling over them on a mountain side, breaking twenty-three ribs plus collapsing a lung, much less return as quickly as possible to the outdoors. If he created myths, the reality did not stray so far from the myth....

Murphy's conclusion is a gratuitous slam at Douglas for being buried at Arlington National Cemetery under false pretenses. Here Murphy stretched the data too far by concluding that Douglas was not a private in the United States Army during World War I because he was merely in the equivalent of the ROTC at Whitman College. But Douglas was, as he claimed, inducted in and honorably discharged from the United States Army and the relevant documents are in both the Library of Congress and the Cravath law firm file on Douglas. Douglas also did not claim, as Murphy implies, that he served in France.

Powe also defends Douglas's protecting the First Amendment rights of Communists, his "cleaning up Wall Street" in the 1930s, and his "prescient environmentalism."