H. Rider Haggard 
She: A History of Adventure [EPUB ebook] 

A young Cambridge University professor, Horace Holly, is visited by a colleague, Vincey, who reveals that he will soon die and proceeds to tell Holly a fantastical tale of his family heritage. He charges Holly with the task of raising his young son, Leo and gives Holly a locked iron box, with instructions that it is not to be opened until Leo turns 25.