Below is the excerpt from page 361 of Altai-Himalaya by Nicholas Roerich. A more poetic version of the sighting also appears in Shambhala 1930 by Nicholas Roerich, page 249.

..............On August fifth--something remarkable! We were in our camp in the Kukunor district, not far from the Humboldt chain. In the morning about half-past nine some of our caravaneers noticed a remarkably big black eagle flying above us. Seven of us began to watch this unusual bird. At the same moment another of our caravaneers remarked: "There is something far above the bird." And he shouted in astonishment. We all saw in a direction from north to south, something big and shiny reflecting sun, like a huge oval moving at great speed. Crossing our camp this thing changed in its direction from south to southwest. And we saw how it disappeared in the intense blue sky. We even had time to take our field glasses and saw quite distinctly the oval form with shiny surface, one side which was brilliant from the sun.

As for English references concerning the Tunguska explosion, I would recommend The Fire Came By 1976 by John Baxter and Thomas Atkins (if you can find a copy).

Another rare one: Giant Meteorite 1966 by E.L Krinov pages 125-165.

And I have to mention a 1980 TV episode of Clark's Mysterious World entitled "The Great Siberian Explosion" because of all the cool film footage it shows from Leonid Kulik's expeditions.

Finally, a comprehensive and up to date account of Russian scientific study of the Tunguska Explosion from 1908 to the present--in English: The Tunguska Mystery 2009 by Vladimir Rubtsov.

As for webpages, I would currently recommend
N.V.Vasilyev

Virtual Exploration Society: Leonid Kulik

A few more interesting facts about the Tunguska explosion:

I added a few things here after reading Mr. Rubtsov's book, highlighted in blue text.

Shortly after Kulik's third expedition in 1929, which lasted some 18 months, a colleague of his named Evgeny L. Krinov--who accompanied him on the expedition--proposed a south-eastern trajectory for the Tunguska object. His proposal was bolstered some 30 years later when the butterfly pattern of forest fall was fully realized. Even though the axis of the blast pattern pointed to an even sharper east to west trajectory than Krinov had suggested, it would seem the exploding meteor theory had reached a point of fruition. However, according to the late Nickolai Vasilyev, when those witnesses who lived east of the epicenter--near the town of Preobrazhenka--were subsequently interviewed, they apparently all reported a low flying object that passed over in the afternoon (par for the course).

Krinov might have gotten it right for all I know, but, again, without the ability to review all of the witness statements, I can't really draw any definite conclusions of my own regarding how well a certain trajectory might fit the witness statements. Really, I could do so much more with a comprehensive translation of the witness accounts. Common sense would say that there must have been some good reason why an aerodynamics professor (Zigel) would be so inclined to choose a southern trajectory over a south-eastern one, and then try to rectify things by suggesting the object made an elaborate, last-minute course change, but who knows? Indeed, it appears that more than one Russian scientist has struggled to explain some major discrepancy in the witness statements. Felix Zigel, by the way, is the person who originally pointed out the unlikelihood of anyone hearing noises made by the object as they watched the object travel. A Russian scientist named Andrei Zlobin, apparently inspired by a particular eyewitness account, has suggested that the trajectory of the Tunguska object actually curved from an easterly one to a southerly one, and that the offset blast pattern--usually attributed to the object's weaker ballistic wave (shock wave) in conjunction with the much more powerful blast wave--was due to complex gas thermodynamics. (If you can understand what he is saying, let me know--it's all over my head.) He also is a proponent of the idea that the object passed directly over the trading post of Vanavara, thereby accounting for the intense heat that people felt there. However, one Vanavara witness clearly describes the heat blast as coming from the north--not from above. As you can see, it's difficult to arrive at a scenario that fits perfectly, especially in regards to the witness accounts.

No meteorite fragments of any appreciable size have ever been recovered. However, microscopic fragments have been found in the soil and trees which could have come from a meteor that caused the Tunguska explosion, although the amounts are barely above the normal dispersion of space dust. According to Mr. Rubtsov, all of the extra space dust in the entire forest fall area put together might equal a ton (TTM page 277). When you consider the fact that the Tunguska object, if it was indeed a 150' wide stony meteor, would have weighed many tens of thousand of tons even if it weighed in on the lighter side of meteors, one ton of material is really nothing at all.

There has been some rather unconventional theories put forth over the years regarding the Tunguska event: The first such theory was put forth in 1946 by a Russian scientist named Alexander Kasantsev. Kasantsev--in the form of a science fiction story--speculated that the Tunguska blast was the explosion of malfunctioning alien spaceship after he noted simularites between the Tunguska blast and the atomic bomb blasts over Japan. Since then, other unconventional theories include an "anti-rock" composed of anti-matter, or even a black hole. An especially wild theory has been put forth recently that the Tunguska object was "chased down" and detonated by an Earth based meteor defense installation constructed by aliens. Hidden alien installations aside, apparently, the author or authors are counting on a lack of common sense in the reader that once a meteor surpasses a certain range in size/mass, attempting to destroy it once it has entered the atmosphere is not really going to accomplish much and--in the case of the Tunguska "meteor"--would probably have just made matters worse.

Despite an enormous blast area that leveled some 1000 square miles of forest, the main force of the blast somehow managed to narrowly miss even the smallest of human settlements and did not result in the immediate death of a single human being; all of this while leaving a nicely detailed record of the event, small remnants of which can still be seen to this very day.

The eyewitness accounts, although open to some interpretation, seem to suggest that during, or immediately after the object flattened into a saucer shaped black cloud, a peculiar "shaft" of light or flame shot from the cloud. Described thusly: Kirensk--"A fiery pillar in the form of a spear"; Nizhne Karelinsk--"Turned into a fiery pillar and disappeared in a moment"; "A forked tongue of flame broke through the cloud"; Krasnoyarsk newspaper 1908--"A huge flame shot up and cut the sky in two"; Vanavera--"The sky was split in two."

This description does not seem to be of the object in flight or of the subsequent mushroom cloud that no doubt followed the explosion. Indeed, there are a number of historical UFO sightings that use the sword or beam analogy to describe some aspect of the sighting. Some currently misunderstood aspect of large meteors? Or something else?

The number of witness accounts that have been translated into English are fairly limited as of the time being. These two websites stand out as having a little more than the usual scrap of information about the witness accounts. The first one also contains another article written by Nicholai Vasilyev (pronounced va-si-ly-ev) from 1992.
Vurdalak | The Witnesses of the Tunguska Event

Tunguska event of 1908 was to be of geophysical origin!

I have come across some disinformation on the web that high levels of ionizing radiation were found near the epicenter of the Tunguska blast. Although there is "circumstantial" evidence that a blast of high energy radiation occured at Tunguska in 1908, including--but not the least of which are--plant growth anomalies, no high energy radiation or indisputable evidence of a high energy radiation event has ever been found in association with the Tunguska blast. The first measurements were not taken until the late 1950s however, about fifty years after the event. Mr. Rubtsov disputs this, stating that the 1959 expedition did find a pattern of radiation that increased towards the center of the forest fall, a fact that would rule out the slightly elevated levels found as being caused by recent nuclear fallout (TTM page 101). At least one witness account I have collected does mention how some individuals who lived near the forest fall became sick that winter after the explosion and dealt with itchy sores, some even dying from the illness.

Magnetic disturbances recorded at the Irkutsk Observatory on June 30th, 1908 strongly resemble those caused by atmospheric nuclear explosions, with the exception of a 6 minute + lag time between the explosion itself and the start of the disturbances. This lag time was probably due to the time it took for the fireball to rise into the upper atmosphere (TTM page 172). The disturbances lasted for about 4 1/2 hours. Atmospheric nuclear explosions cause a distinctive magnetic disruption for a number of reasons, which, according to some scientists, the Tunguska blast should not have been able to produce. Although meteors are capable of causing magnetic disruptions as they pass through the ionosphere, no disruption by a meteor that compares with the Tunguska blast has ever been recorded.