It's fruitcake time. No matter how many books pile the shelves nearly six decades after Nazism burnt in its final flames, there are still chambers of horrors left to open. Christopher Hale returns to Auschwitz in the end, sure enough, but his centrepiece is the bizarre all-German expedition to Tibet dispatched by Himmler to hunt for the remnants of the Aryan race. Call it Raiders of the Lost Lama.

Here's Ernst Schäfer, the most brilliant German explorer of his day (or morning, since this famous veteran of two epic Tibetan trips is only 28). He is a very general sort of scientist, a collector of fauna and flora with charisma to spare. At his side stands Bruno Beger, a very specific sort of anthropologist, enthralled by theories that remnants of the Nordics, the Aryans, the master race he preferred to call Europids, could be found in the lost worlds around Lhasa.

Who strews cash and influence in their path? Heinrich Himmler, head of the SS. He signs them both up as officers. He helps them mount this expedition for the greater glory of the fatherland and the fascination of its Reichsführer. He mixes murder and mysticism in equal measure. Atlantis, the Lost Ark? Himmler loves all that stuff. But mostly, it's racial purity that fascinates him.

One day - a little later in the Beger association - he muses over the prehistoric Venus figurines unearthed all over Europe, with their bulbous backsides. Might these be relics of some older race expelled from our continent when the superior Aryans turned up? And - looking around - were Jews and Hottentots the living representatives of these ancient inferiors?

Just so, says Beger, head of research at the Ahnenerbe, the SS academy of perversion. 'Racial similarities between the Hottentots, the North Africans and the Near Eastern people are unmistakable. Among well developed bodies that could be Jewesses it is noticeable that they have very well-developed bodies which could be linked to the fat-bottomed image seen among Hottentots and bushmen.'

And so our heroes set off for Tibet. There's a good deal of fencing with British bureaucracy. Any fool (except a large and dismaying array of English ones) can see the way the winds of war are blowing. Do we want to give visas to Schäfer and this odd bunch of sub-Shackletons? Slowly, with cunning and courage, the infamous five make it through Calcutta and Sikkim until they're at the heart of the Hidden Kingdom. They film and collect and fornicate energetically with local maidens used to polyandry. They find and shoot sacred goats. But then they hear that war's broken out. So they go home.

If Christopher Hale, writing his first book following two TV documentaries, was setting out to tell a simple, lurid tale, that might feel a bit of a let-down. The months of struggling to mount the expedition and of dodging British authority seem set to provide a climax of derring-do. But, in reality, there is only anticlimax, a story line wandering away down every interesting side alley.

Meet Helena Blavatsky, the nineteenth-century mystic who wove a tapestry of tosh around the name Tibet. Read her bestselling Secret Doctrine - a 'turbid cocktail of bogus Tibetan wisdom and evolutionary science'. Follow the ranting logic of Professor Hans FK Gunther, who taught his pupil Beger to believe that Germany's own stock was becoming perilously mixed, unable in its impurity to sustain high civilisation. Himmler, in this context, becomes a nut among nuts, an unsuccessful small farmer whose lurid imagination turned him into the grimmest reaper of the Reich. Another part of the jigsaw falls into place.

Was Schäfer himself a demented racist? Not at all. He was, for the most part, a dogged searcher with a wanderlust. The specimens he brought back from Tibet by the crateful expanded human knowledge. He gave Beger's grottier theories a wide berth. Schäfer could have emigrated to America after his first two expeditions, but Himmler seduced him with funds and flattery. He sold out, like thousands of young academics, for the opportunity to work and prosper. Joining the SS, for him, was only a means to an end.

Beger - still surviving, far into his nineties, when Hale saw him - belonged to a different clan. He was a believer, a searcher after his own version of scientific truth. And where did that search lead him and the other Ahnenerbe scientists? To Auschwitz, where countless skulls were measured and countless death masks made in the effort to sort the master races from the chaff.

Suddenly Schäfer's adventuring slides into something malignant, not merely macabre. Perhaps there was no single rationale for the concentration camps. They were a mix of extermination, administrative convenience and demographic re-balancing. But science - this 'science' - was part of the brew. And it is difficult, tracing its growth from cheap faction to university subject, not to shudder over the detail of derangement.

Hale sometimes writes a trifle lumpily. He is, maybe, too fond of saying what 'must' have been going through some character's head at any given time - a journalist's trick which undermines his scholarship. Occasionally his habit of cross-cutting between Tibetan hillsides carpeted with flowers and menacing events far away in Europe is Channel 4 voiceover, not easy narrative.

But even so, the quality of information and effort compels respect. Himmler's Crusade is a stew of delusion and dreams and dementia: a cheap paperback of poisonous theory peddled to the credulous, the fearful and the calculating. Once, towards the close of their collaboration, Beger wrote to Schäfer, describing the 'tall, healthy child of nature,' he'd been experimenting on. 'He could have been a Tibetan. His manner of speaking, his movements and the way he introduced himself were simply ravishing; in a word, from the Asian heartland.' And then the ravishing child was dead, dismembered; a whole new definition of terrible beauty.