Wikipedia reports that "The book was first published in September 1885 amid considerable fanfare, with billboards and posters around London announcing "The Most Amazing Book Ever Written". It became an immediate best seller. By the late 19th century explorers were uncovering lost civilizations around the world, such as Egypt's Valley of the Kings, and the empire of Assyria. Africa remained largely unexplored and King Solomon's Mines, the first novel of African adventure published in English, captured the public's imagination."
It was also the best selling book of 1885 and is arguably the first of the "Lost World" genre of fiction. I love the book's dedication, which reads:
This faithful but unpretending recordof a remarkable adventureis hereby respectfully dedicatedby the narrator,ALLAN QUATERMAIN,to all the big and little boys who read it.
And of course any novel with a Naval Officer named "Captain Good" cant be bad at all!