In the end credits, you can see the words "From an original story by Scott O'Dell". Indeed, to write the screenplay, Bernard Deyriès and Jean Chalopin were inspired by this book: The King's fifth. Personally, I think that there is little connection between the series and the book! Here is a brief summary.
Scott O'Dell is a contemporary American author who wrote many books for children. His stories often take place in a historical context and he likes to describe great adventures and characters with strong temperaments. He lived in Southern California near the Mexican border: in most of his books, there are Spaniards, Indians or Mexicans.
Scott O'Dell was born in 1899. He is the son of a railway employee residing in Los Angeles. He wrote his first children's book, The Island of Blue Dolphins in 1960. The book The King's Fifth, whose series The Mysterious Cities of Gold comes from, was written in 1966. Scott O'Dell received an International Andersen Award in 1972 for all of his works for children. He died in 1989. The original novel by Scott O'Dell was published in France by Flammarion in the collection Castor Poche, under the name La Route de l'Or. It is a book of 464 pages but quickly read.
The story is very different from the series. Some characters do not exist, including Tao and all the book is much more realistic than the series: no Great Condor, no fabulous mechanisms and no Olmec people. The story could really have taken place during the 16th century as described in the book: Esteban is a young cartographer who is in charge to map uncharted territories discovered by a group of Spaniards. They hope to find cities of gold, accordind to an Inca legend. He will meet Zia, a young guide who acts as a translator. Esteban eventually follows Mendoza, who wants to attempt the journey alone, without the soldiers of Pizarro. But after a long and perilous journey, they will only find a few pebbles of gold in a lake where a ritual is performed. But no City of Gold! Esteban and Mendoza manage to steal the gold by emptying the lake and they flee to escape the furious inhabitants. The return will be difficult: Mendoza and his men die (Mendoza dies stupidly, bitten by the dog of Gomez). Esteban managed to reach a port, after hiding the gold. He is then arrested and jailed for failing to declare his gold to the King of Spain. This is also from his jail - where he awaits his trial - that Esteban tells us this adventure. The character of Esteban is very different between the book and the series. Zia is a thief who looks like a tomboy! And Tao doesn't exists ...
Note that in 2013, with the broadcast of season 2 and the manga, Kazé Editions published a new French edition of the novel, with a new cover on which the reference to the series is clearly highlighted as you can see!