King Solomon's Mines
I've read this book three times now: once in undergrad, once for a seminar in grad school, and now for my exams. That's a lot of ol' Haggard, that Victorian adventure beast.
At the time it was published, this book was advertised as "The Most Amazing Book Ever Written," which is pretty hilarious, but the fact that it was a publishing phenomenon unlike anything else at the time kind of lends it some credence. It was like the Harry Potter of 1885. If you read at all (meaning, if you had money and an education), you would have read this book. And none of Haggard's other (many, many) books even came close to matching its popularity.
I had the good fortune to read this book with the #1 Haggard scholar in the world (a weird honor to have, I think), who just happens to work at my university. He freaking loves this book. He loves to talk about how this is the most influential novel that nobody has read.
And what is the legacy of King's Solomon's Mines? I can think of four things right off the bat:
1) J.R.R. Tolkien was a huge fan and almost certainly took inspiration for the creation of Gollum from Haggard's ancient and evil Gagool,
2) Allan Quatermain lives on in several places, most notably Alan Moore's The League of Extraordinary Gentleman,
3) Quatermain is also understood to be the template for the character of Indiana Jones, and
4) Haggard has provided Noah Wyle with something to do in his post-ER days, with the lovely and cheeserific second film in the Librarian series: Return to King Solomon's Mines.
For this last thing, I am particularly thankful. Noah Wyle is adorable.