Pellucidar (Pellucidar Book 2) by Edgar Rice Burroughs - Even better than the first book.

Kindle edition of the second book in Burroughs classic series. This edition includes an active table of contents.

From Wikipedia:
David Innes and his captive, a member of the reptilian Mahar master race of the interior world of Pellucidar, return from the surface world in the Iron Mole invented by his friend and companion in adventure Abner Perry.

Emerging in Pellucidar at an unknown location, David frees his captive. He names the place Greenwich and uses the technology he has brought to begin the systematic exploration and mapping of the unknown land while searching for his lost companions, Abner, Ghak, and Dian the Beautiful. He soon encounters and befriends a new ally, Ja the Mezop of the island country of Anoroc; later he finds Abner, from whom he learns that in his absence the human revolt against the Mahars has not been going well.

Even better than the first book.
This is the sequel to At The Earth's Core. Some of the reviews here say that it isn't quite as good as the first. I happen to think that it's a little bit better. Burroughs seems to avoid some of the continuity errors he made in the first book, and really it's quite a worthy addition to any adventure-reader's library.

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