Animorphs #1-7
Megamorphs #1: The Andalite's Gift 
Animorphs #8-12
The Andalite Chronicles
Animorphs #13-18
Megamorphs #2: In the Time of Dinosaurs
Animorphs #19-22
The Hork-Bajir Chronicles
Animorphs #23-29
Megamorphs #3: Elfangor's Secret 
Animorphs #30-35
The Visser Chronicles (Also known as just 'Visser') 
Animorphs #36-40
Megamorphs #4: Back to Before
Animorphs #41-47 (Note: Starting with #45, the series begins its final arc to end) 
The Ellimist Chronicles
Animorphs #48-54