Although this book had an interesting story to tell, I had some issues with the way Martin chose to tell it.
For starters, the structure of the book was a little off-putting. He alternated chapters concerning the journey and fate of the hair with chapters about Beethoven's life, and I found that kind of jarring, somehow. I also found that he repeated himself a lot, and went into greater detail about seemingly minor incidents than felt necessary. Frankly, a lot of it felt like filler. This book could easily have been half the length, or perhaps just a feature article in a decent magazine.
One such example is the whole Gilleleje story. Denmark and its people were truly heroic during WWII, I will acknowledge. The story of how, as a country, they accepted Nazi rule up to a point, but when the line was crossed, pretty much unanimously stood up and said "No way. Not cool," and proceeded to help get hundreds of Jews out of the country to safety in Sweden is remarkable and really inspiring. It's the kind of story that helps restore your faith in humanity after so much else in the world shatters it, and I'm glad to have been made more aware of it. However, I think it was given more importance in this story than it really needed. I was particularly vexed by the oft-repeated notion that the lock played some kind of key role in the Gilleleje escape. Yes, it somehow made its way there, and once there, was given to the doctor, but to say that it played a significant role almost implies that Dr. Fremming wouldn't have helped otherwise, or maybe even would have deliberately hindered, and I just don't think that's true. He was there, he was helping, and someone happened to give him this thing. I understand that it's a step in the hair's journey, and that understanding that step would be very interesting to those directly involved with the hair, but I feel like it was given more importance than it really merited, simply because it's a mystery.
And finally, I found his writing style odd. For one thing, he seemed to feel it was necessary in the "history of the hair" chapters to use the pluperfect tense. I don't know why he felt this way, but I think it was a bod choice. For starters, it adds unnecessary complication to all your sentences, and usually renders them less clear, so that's a mark against it. It also leaves you with nowhere to go when you want to refer to something father back than your baseline. And it's one of those tenses that is hard to use consistently when it's your main tense. Some verbs and sentence constructions just sound very weird with that tense, and Martin got around that by simply using the simple past in those cases. Which is inconsistent, and frankly, just plain sloppy writing.
His use of adverbs is also unusual. I understand that there are those who are fanatical supporters of the ideal of no split infinitives, and that's fine. But there's no cause I'm aware of not to split the auxiliary verb from the main one. In fact, the Oxford Online specifically states that the adverb should fall between the auxiliary verb and the main verb. I don't have a US style manual handy (Chicago online requires a subscription), so I don't know what the deal is there, but in any case, my point is simply that Martin's insistence on putting his adverbs before his auxiliary verbs is weird and awkward to read, making me stumble pretty much every time I came to one.
The upshot is that while this book and its subject matter were interesting, I wish they'd been tackled by a better writer.