I remember watching a movie based on this book starring Richard Chamberlain. I actually quite enjoyed the film, though one of the major differences that I discovered between the film and the book is the inclusion of a beautiful white female. I guess that is what one really has to expect from Hollywood, particularly since there have been a lot of Hollywood movies that have been based on books of old and they have thrown in a girl because, well, a Hollywood movie just isn't a Hollywood movie unless there is a girl for the protagonist to fall in love.
As for the book, well, it was clearly an adventure story written for the younger audiences and certainly not a post-colonial commentary in the vein of books such as Joseph Conrad's The Heart of Darkness. In fact there is pretty much no social commentary in this book at all, rather it is designed to appeal to people's lust for adventure and exploring the deep and unknown areas of the world as they were at this time.
When Haggard was writing, much of the world had already been mapped, though there were still large sections of the African, South American, and Asian interior that remained places of mystery. However, despite them being unexplored, the general attitude of the colonial powers at the time was to simply claim those areas and explore them later. I believe, at least with Africa, that the land that was claimed along the coast pretty much extended inland until it pretty much hit the centre, which is why you basically see a patchwork of countries in Africa which are not necessarily divided along tribal lines. As the colonial powers began to penetrate the interior of the continent, they would then set up their own claims on this land.
However, as I advised, this book has nothing to do with colonial expansion, and everything to do with the exploration of a dark continent in search of a mythical gold (or diamond) mine. The story of King Solomon's mines goes back to the Old Testament (and it is interesting that these early stories still have Christian references attached to them) where there is reference to mines in the land of Ophir from which great wealth was brought to Jerusalem to establish Solomon's rule. Personally, I have no idea where Ophir is supposed to be located, though I suspect modern scholarship has more understanding of it than did Haggard.
The book itself was okay: not that great and not that exciting. In a way I preferred the movie because, well, it was an Indiana Jones style adventure movie. I guess the book was trying to be like that as well, and I suspect that Spielberg was influenced somewhat by the writings of Haggard when he created Indiana Jones. It is also interesting to finally read the book that ended up inspiring that movie that I quite enjoyed watching.
However, if you are expecting Richard Chamberlain (and I simply cannot picture Alan Quartermain not being picture Richard Chamberlain):
to go hacking through jungles, running away from giant boulders, and solving puzzles in an ancient mine, well, you do get some of that, but the bulk of the book seems to be sent in the land of some lost African tribe and how Quartermain and his cohorts act to change their entire culture and to establish a king that will be more British than barbaric. In fact they seem to go to war against those members of the tribe that will not accept and adapt to the British way of life. In a way it is reminiscent of the colonial era when the colonists would come along and force the inhabitants of the land to become British, and if they did things that the British found repugnant then they would be forced to stop.
In fact, that still happens today in our enlightened culture. These days this is called human rights. For instance, if a culture, who has been doing things a certain way for hundreds of years (such as forcing women to wear veils) is seen by the enlightened academia of the Western World as being oppressive, then the enlightened academia believes that it is their right to go in there and stop them from doing that. Look, I am very much in favour of human rights, and very much against the oppression of women and other minorities, however, let us not criticise the British for interfering in the culture of the colonised and claim that they were being imperialist when we on the left are doing exactly the same thing ourselves.