In my essay WHAT THE WELL-DRESSED SUPERHERO OUGHT TO WEAR I wrote:
In the pop-culture world as we know it today, there are a lot of characters who have superhero-like powers, weapons or adventures, and who wear commonplace attire. James Bond may be the foremost example of this type, and there's no doubt that the prose novels qualify as adult-oriented pulp. However, Bond's enormous popularity across many cultures stems principally from the movie adaptations, which may have caught fire from being culturally "in the right place at the right time." Before Bond, popular fiction-- prose fiction, movies, comic strips-- played host to innumerable characters who wore ordinary clothes but enjoyed extraordinary adventures, whether they chased down weird masterminds (Doc Savage), mystic menaces (Jules De Grandin, Mandrake the Magician), or just freaky-looking criminals (Dick Tracy).
By my lights, the character in serial pop-fiction who represents the earliest flowering of the superhero idiom is Rider Haggard's Allan Quatermain. I've not investigated every 19th-century claimant to the status of "first superheroic type," but my general feeling is that most of them tend to be "one-off" concepts, like THE BLACK MONK from 1844. Not only was KING SOLOMON'S MINES wildly successful as a stand-alone book, its repute lasted long enough that Haggard continued to add adventures to his protagonist's history, though most were prequels, since the author killed off the character in the second book, ALLAN QUATERMAIN. Quatermain himself possesses no special powers or weapons, but he does encounter a smattering of weird opponents. The antagonists of the second Quatermain novel, for instance, come from the city of the Zu-Vendis, a white-skinned people whose ancestors colonized a part of Africa in archaic times and continued their customs uninterrupted into modern times. Thus Haggard invented "the lost race novel," which Edgar Rice Burroughs and others further promulgated.
KING SOLOMON'S MINES does not precisely concern a "lost race," though it does conform to the uncanny version of the trope I call "exotic lands and customs." Purportedly Haggard wrote MINES as a response to the popularity of Stevenson's TREASURE ISLAND. But whereas everyone in the Stevenson book is anxious to claim the titular treasure for pure gain, only one Haggard character goes to Africa to make his fortune by finding the fabled mines of King Solomon-- and when he goes missing, white hunter Quatermain is hired to search for the missing man by Sir Henry Curtis, brother of the treasure-seeker. During their trek across a hostile desert, Quatermain and Curtis are accompanied by two other major characters: Captain Good, who serves more or less as comedy relief, and a mysterious Black African named Ignosi. When the four men succeed in crossing the desert, they find themselves in the isolated domain of the Kukuanas, a Zulu-like people. Because these Africans have been cut off from modern developments, they have a superstitious feeling toward the three white men, though the Kukuana's tyrannical leader Twala is quite willing to chance killing these interlopers. The white men find themselves drawn into a dynastic struggle when they learn that Ignosi's true name is Umbopa, and that despite an exile from this land since childhood, he, not Twala, is the rightful ruler of the land.
What makes the Kukuanas uncannily exotic is not their isolation, though, but their custom of witch-finding. Chief Twala has no problem with sacrificing dozens of tribespeople to the gods when his henchwoman Gagool fingers her victims as witches. Gagool, an incredibly ancient woman, claims to have lived for centuries (making her a figurative predecessor to Haggard's creation She-Who-Must-Be-Obeyed) and claims that she can only be killed by accident, though both of these claims are left ambivalent. Even after the white adventurers help Umbopa's loyalists overthrow the reign of Twala, they almost meet their doom when they force Gagool to lead them to the mines of Solomon, marked by statuary from Biblical times. Though Gagool meets her end here, she's weird enough that I deem her as much an ancestor of the "supervillain" as Quartermain is to the "superhero."
Naturally, there are aspects of MINES that could never pass the political correctness test today. Yet it is not a racist novel, or even what some pundits would call a "white savior novel." (The white guys score some points against Twala's forces, but in the end the victory is that of the loyalists allied to Umbopa.) Umbopa is also a heroic personage, probably the first black hero of English-speaking literature (if one uses "hero" according to the combative mode I've outlined here). In the second Quatermain novel, Haggard introduces Umslopagaas, a Zulu hero who's a support-character in that novel but later gets his own solo story, NADA THE LILY, which I've not yet read.
Not all novels called "classics" deserve that title. But like SHE, KING SOLOMON'S MINES richly deserves the accolade.