The “Magic Tree House” series by Mary Pope Osborne is still a best-seller, and a perfect summer read for those entering 2nd or 3rd grade. Here are some other transitional novels with a sci fi or mystery element:
Abbott, Tony. Secrets of Droon series
Adler, David. Cam Jansen series
Alexander, Lloyd. Time Cat
Conrad, Pam. Stonewords
Eager, Edward. Half Magic and sequels
Greenburg, Dan. The Zack Files series
Greer, Gerry. Max and Me and the Time Machine
Gutman, Dan. Baseball Card Adventures series
McAllister, Margaret. Ghost at the Window
McMullan, Kate. The Dragon Slayers’ Academy series
Meacham, Margaret. Quiet! You’re Invisible
Osterweil, Adam. The Comic Book Kid
Preller, James. Jigsaw Jones series
Ransom, Candice F. The Time Spies series
Roy, Ron. A to Z Mysteries series
Scieszka, Jon. The Time Warp Trio series
Skurzynski, Gloria. National Parks Mysteries series
Stilton, Geronimo. Geronimo Stilton series
Winthrop, Elizabeth. Castle in the Attic
Woodruff, Elvira. George Washington’s Socks