Sannikov Land by Vladimir Obruchev, 1926.
If you like lost world novels, I guarantee that this obscure Russian classic will press all your buttons. There are encounters with prehistoric megafauna, beautiful and willing savage women, war between stone-age tribes, weird shamanistic rites, earthquakes and volcanic eruptions, and a boy's own enthusiasm for bagging big game. It's true that the characters are indistinguishably wooden mouthpieces for the author's opinions and the plot is pure pulp, but those faults are redeemed by the novel's rigorous scientific sensibility.
Obruchev was a geologist and academician, high in the former USSR's scientific hierarchy (amongst other things he had a mineral, a mountain, and a crater on the Moon named after him). His descriptions of the harsh beauty of the Russian Arctic Circle, and of the privations experienced by his explorers, are crammed with telling detail; given the abundance of frozen mammoths in Siberia, one suspects that he may have been drawing on experience when recommending roast mammoth trunk as a particular delicacy. There are lyrical infodumps about geology and prehistoric fauna; the lost land, nestled in a vast Arctic volcano, is drawn with evocative vermisilitude.
Sannikov Land has been long out-of-print -- the edition I have is an English translation published in 1955 by the Foreign Languages Publishing Association of Moscow -- and as one of a series of 'Soviet Literature for Young People', it was a small part of the former USSR's Cold War arsenal. When it was published, it was probably illegal, or at least ever-so slightly dangerous, to own it in the USA, so it may be hard to find. But believe me, the search will be worthwhile. I'm off to look for Obruchev's other scientific romance, Plutonia. It's a hollow-earth story, and I can't wait to read it.*
*I eventually tracked down a copy owned by China Mieville. He hadn't read Sannikov Land, so we made an equitable trade.