If you like the Pendragon series by D.J. MacHale
I am glad that you enjoyed Pendragon.
Now you will want to continue through the series.
The next book is
Pendragon The Lost City of Faar.
In this second adventure in the Pendragon Quartet, readers find 14-year-old Bobby Pendragon traveling through the "flume" to Cloral with his Uncle Press, in pursuit of Saint Dane. This waterworld with a vague Australian/Atlantis air will likely intrigue fans who enjoyed the aquatic world of Jules Verne's 20,000 Leagues Under the Sea and the teamwork escape aspect of Journey to the Center of the Earth. Bobby is still uncertain about what happened to his family after he was recruited as a "Traveler," a heroic role that first pitted him against the shapechanging monster, Saint Dane, in the series' debut, The Merchant of Death. It's a radically different life from that of junior high student in Stony Brook, Conn., where his friends Mark and Courtney eagerly await the journals in which he updates them with the latest events from various colorful "Territories." In the fabled city of Faar, beneath the waves, Bobby meets Spader, who is also a Traveler, and quickly becomes his first mate. Unbeknownst to Spader, his submerged city is partially responsible for the peace felt above the surface in the floating habitats. When Saint Dane decides to annihilate Faar, Bobby and his Traveler buddies must race to outwit the villain. MacHale embellishes his science fiction with just enough silly touches to leaven the mood; for instance, when the magic ring that Bobby gave Mark twitches (which means it's about to "deliver" Bobby's journals), it expands so that Mark must remove it, and the glowing stone transforms into a "black hole," spewing musical notes and light.
The Never War #3 .
Fourteen-year-old Bobby Pendragon is a loyal friend, sports star, devoted pet owner -- and Traveler. Along with his uncle Press, Bobby has visited the alternate dimension of Denduron and participated in a civil war. He's also waded through the endangered underwater territory of Cloral. Now Bobby once again finds himself thrust beyond the boundaries of time and space into a place that seems somewhat familiar: First Earth.
Bobby and the Traveler from Cloral -- Spader -- have flumed to New York City, 1937. Against a backdrop of gangsters, swing music, and the distant sound of a brewing war, the two must uncover the evil Saint Dane's newest plot. But is Bobby ready for the difficult choices ahead?
The Reality Bug #4.
The territory of Veelox has achieved perfect harmony. Fifteen-year-old Bobby Pendragon arrives on this territory in pursuit of the evil Saint Dane, but all is peaceful on Veelox -- because it's deserted. The inhabitants have discovered a way to enter their own personal dream worlds, where they can be whoever they want, wherever they want. Their bodies lie in stasis while their minds escape to this dream realm.
Fresh from his battle with Saint Dane in 1937 Earth, Bobby is confident that they can defeat whatever Saint Dane has planned for this world. But once Bobby enters the virtual world will he be able to resist the lure of the ultimate in escapism?
Black Water #5.
The Rivers of Zadaa#6.
The Quillan Games #7.
The Pilgrims of Rayne #8.
Raven Rise #9.
If you have finished that series and want to start on another sci-fi series try
The City of Ember by Jeanne Du Prau.
It is always night in the city of Ember. But there is no moon, no stars. The only light during the regular twelve hours of "day" comes from floodlamps that cast a yellowish glow over the streets of the city. Beyond are the pitch-black Unknown Regions, which no one has ever explored because an understanding of fire and electricity has been lost, and with it the idea of a Moveable Light. "Besides," they tell each other, "there is nowhere but here" Among the many other things the people of Ember have forgotten is their past and a direction for their future. For 250 years they have lived pleasantly, because there has been plenty of everything in the vast storerooms. But now there are more and more empty shelves--and more and more times when the lights flicker and go out, leaving them in terrifying blackness for long minutes. What will happen when the generator finally fails?
Twelve-year-old Doon Harrow and Lina Mayfleet seem to be the only people who are worried. They have just been assigned their life jobs--Lina as a messenger, which leads her to knowledge of some unsettling secrets, and Doon as a Pipeworker, repairing the plumbing in the tunnels under the city where a river roars through the darkness. But when Lina finds a very old paper with enigmatic "Instructions for Egress," they use the advantages of their jobs to begin to puzzle out the frightening and dangerous way to the city of light of which Lina has dreamed.
That series continues with The People of Sparks, The Prophet of Yonwood, and The Diamond of Darkhold all by Jeanne DuPrau.