The Land That Time Forgot. by Edgar Rice Burroughs

The Land That Time Forgot opens with the discovery near Greenland of a floating thermos flask containing a manuscript by castaway Tyler Bowen, Jr. The document recounts a series of adventures that starts with a sea battle against a German U-boat and ends on a mysterious island populated by hostile prehistoric animals and people. The second part of the book, "The People That Time Forgot, " continues the story with the tale of Tom Billings, who has been sent on a mission to rescue Bowen after his manuscript was discovered.
He flies solo over the mountainous cliffs that encircle the island and is attacked by a monstrous flying reptile, forcing him to crash-land. Billings then attempts to make his way on foot back to the rest of his party while contending with dangerous inhabitants from different stages of human development. The final installment of the story, "Out of Time's Abyss, " reveals what happened to Bradley, a crew member who was sent on a scouting expedition earlier in the story and was never heard from again.
This trilogy of short novels was originally published serially in 1918 in Blue Book Magazine. In 1924 they were published in a single volume by A. C. McClurg. The Burroughs fan community seems to fall into two camps about whether the story comprises three separate novellas, or whether it's a single novel divided into three parts.