Magic Tree House Series by Mary Pope Osborne

Dinosaurs Before Dark (Book 1) Knight at Dawn (Book 2) Mummies in the Morning (Book 3) Pirates Past Noon (Book 4) Night of the Ninjas (Book 5) Afternoon on the Amazon (Book 6) Sunset of the Sabertooth (Book 7) Midnight on the Moon (Book 8) Dolphins at Daybreak (Book 9) Ghost Town at Sundown (Book 10) Lions at Lunchtime (Book 11) Polar Bears Past Bedtime (Book 12) Vacation Under the Volcano (Book 13) Day of the Dragon King (Book 14) Viking Ships at Sunrise (Book 15) Hour of the Olympics (Book 16) Tonight on the Titanic (Book 17) Buffalo Before Breakfast (Book 18) Tigers at Twilight (Book 19) Dingoes at Dinnertime (Book 20)