From InfoPlease:
- The Little Prince (Le Petit Prince), Antoine de Saint-Exupéry (1943) 140 Million
- Harry Potter and the Sorcerer’s Stone, J. K. Rowling (1997) 120 Million
- Alice’s Adventures in Wonderland, Lewis Carroll (1865) 100 Million
- The Lion, the Witch and the Wardrobe, C. S. Lewis (1950) 85 Million
- The Adventures of Pinocchio, Carlo Collodi (1881) 80 Million
- Harry Potter and the Chamber of Secrets, J. K. Rowling (1998) 77 Million
- Harry Potter and the Prisoner of Azkaban, J. K. Rowling (1999) >60 Million
- Harry Potter and the Goblet of Fire, J. K. Rowling (2000) >60 Million
- Harry Potter and the Order of the Phoenix, J. K. Rowling (2003) >60 Million
- Harry Potter and the Half-Blood Prince, J. K. Rowling (2005) >60 Million
- Harry Potter and the Deathly Hallows, J. K. Rowling (2007) >60 Million
- Heidi, Johanna Spyri (1880) ~50 Million
- Anne of Green Gables, Lucy Maud Montgomery (1908) ~50 Million
- Black Beauty, Anna Sewell (1877) ~50 Million
- Charlotte’s Web, E. B. White (1971) ~50 Million
- The Tale of Peter Rabbit, Beatrix Potter (1902) 45 Million
- The Very Hungry Caterpillar, Eric Carle (1969) 43 Million
- The Wind in the Willows, Kenneth Grahame (1908) ~25 Million
- Where the Wild Things Are, Maurice Sendak (1963) ~20 Million
- Charlie and the Chocolate Factory, Roald Dahl (1964) ~20 Million
Link to the rest at InfoPlease
PG notes that various compilers of best-selling book lists don’t always use the same sources, metrics (number of copies sold vs. number of dollars, euros, etc., earned) or ranking process.
The OP doesn’t include an “of the 20th century” qualifier, but is instead “all time.”