Rory In terms of adventure stories similar I would say Rudyard Kipling's The Jungle Book and Sir Arthur Conan Doyle's The Lost World. (which also came out the same year as this).
Becky First you can read all 25 or so of the Tarzan books, although the later ones are a bit repetitive, then there are all of Edgar Rice Burroughs other series, including the John Carter books set on Mars. When you finally run out of ERB books, I would recommend Doc Savage as another series written about the same time and with a similar style. If you prefer a wilder protagonist, you could try Conan the Barbarian books, although they were never my favorites.