There are 3 books in the Atlantis series: Hidden Courage, Tomb of Atlantis and Curse of Atlantis. Please see recommended order of reading below.

Hidden Courage: is the prequel in the Atlantis series (not essential)
Tomb of Atlantis: is book 2
Curse of Atlantis: is book 3 (sequel)

Curse of Atlantis
In Tomb of Atlantis, Jack Roberts, an adventurer, discovered an artifact that may have belonged to a pyramid contained within the lost city of Atlantis. In Curse of Atlantis, the search for the pyramids continues. Jack and his archaeologist friends, Serena and Javier Arista, plan to take the artifact to Greece in order to find its connection to the lost pyramids of Atlantis. However, prior to leaving on the trip, Serena and the artifact are taken hostage by unscrupulous thieves who only want the riches contained within the pyramids. For Jack and Javier, it is a race against time to discover where the pyramid is, that contains the key to the lost civilization in order to save Serena and the ancient secrets of Atlantis.