Tarzan of the Apes by Edgar Rice Burroughs
One of the most underrated books of all time, this is a great adventure, a great love story, and a brilliant story. Get it for free in a variety of eBook formats here. This link will take you to Feedbooks, but the book is available at Manybooks.net and other perveyors of free books.
City of Amathar 
One of my all time favorites! Your readers might also find some good books to read here:http://universitiesandcolleges.org/walden-online/