Susan Williams seems to have contributed a lot of new and useful research to the matter of Hammarskjöld's death, but she hasn't found conclusive evidence proving that he was assassinated. She does a pretty good job of establishing that white Rhodesians, separatists in the Congo, and some Western governments all had the means, motive, and opportunity to attack or sabotage Hammarskjöld's plane. However, even some of the most suspicious behavior, like the Rhodesian government's apparent tampering with the crash evidence and Lord Alport's misdirection in the hours between the plane's disappearance and the official discovery of the crash site, can probably be explained by incompetence or laziness as easily as by malice.
Meanwhile, Williams barely addresses the physical evidence of the autopsies or the crash investigation except to point out their suspicious omissions and irregularities. A true rebuttal to the official crash investigations, which is what I wanted most out of this book, never materializes. (One chapter ends with a cliffhanger where she wonders aloud whether the plane fragments are still sitting in a hangar at Ndola airport waiting to be examined — which seems unlikely to me — but she never follows up on this. Annoying.)
In a couple places she relies on the testimony of Harold Julien, Hammarksjöld's bodyguard and the sole survivor (for a few days) of the crash. Julien's statements, on their face, support the notion that the plane was shot down or bombed, but Williams spends virtually no time establishing his physical and mental state, the nature of his injuries, or why he eventually died, all of which are crucial to understanding the credibility of his testimony.
It seems that Who Killed Hammarskjöld? isn't meant to be a definitive history that stands on its own, but rather a compilation of historical research that should be compared and contrasted with the official Rhodesian, United Nations, and Swedish inquiries. A reader who comes along expecting a comprehensive treatment of the subject will be disappointed.
The most interesting evidence Williams presents, in my opinion, is the cockpit radio transmission allegedly picked up by the U.S. National Security Agency in Cyprus right at the time when Hammarskjöld's plane went down. In the transmission, an unidentified pilot claims to have shot someone down; this, combined with the testimony of black villagers around Ndola, points pretty strongly towards a deliberate assassination or a hijacking gone wrong. This transmission, however, was picked up in Cyprus as a re-transmission, meaning a transcription made by another listening post, and was conveyed to Williams by one of the Cyprus NSA staffers from memory (or maybe his personal notes) decades after the fact. Neither that staffer nor Williams succeeded in getting hold of the original recording or transcript, assuming it exists.
There and in other places, Williams's research efforts hit a wall of government secrecy. Crucial primary material that would help explain many of the suspicious events are either missing or classified. Further progress in understanding how and why Hammarskjöld died will probably have to wait for those documents to be declassified due to age or whistleblowing.
For all that, the subject matter is interesting enough that I didn't fail to enjoy reading Who Killed Hammarskjöld?; it just wasn't nearly as good as I hoped.