This is a classic adventure story taking the form of a long letter from Alan Quartermain to his brother, in which he narrates his experiences in search of a lost Englishman who was on a quest to find the legendary diamond mines of King Soloman.
It is presented straightforwardly and suffers from being somewhat predictable with regard to the plot, but it shows the prevalent attitudes of its time and location just as well as, say, Tess of the D'urbevilles does its time and location. Some of those attitudes make the book slightly unsettling in an unintentional way, as they are (hopefully) not widespread these days.