You don’t have to be a genius or complete any assignments. All of these books are fairly accessible to the average reader.
I read the majority of them in high school, and they set me (a dyslexic student with undiagnosed adhd) up for a lifetime love of mathematics.
Many of these books could also be categorized as popular science or history books. A couple of them, such as Surreal Numbers and Flatland, are actually fiction!
 Weapons of Math Destruction: How Big Data Increases Inequality and Threatens Democracy
| 3.72 | 106 reads | |
 e: the Story of a Number
| 4 | 5 reads | |
 A Brief History of Infinity: The Quest to Think the Unthinkable
| 2 | 1 read | |
 Four Colors Suffice
| 4 | 1 read | |
 Hexaflexagons and Other Mathematical Diversions
| 0 | 1 read | |
 Flatterland : like flatland only more so
| 3.5 | 8 reads | |
 An Imaginary Tale
| 5 | 2 reads | |
 The Music of the Primes
| 4.33 | 4 reads | |
 The Joy of x
| 4.06 | 19 reads | |
 Our Mathematical Universe: My Quest for the Ultimate Nature of Reality
| 4.05 | 31 reads | |
 Surreal Numbers Surreal Numbers
| 4 | 2 reads | |
 Math Without Numbers
| 4 | 4 reads | |
 The Math Book
| 5 | 2 reads | |
 The Drunkard's Walk
| 3.95 | 44 reads | |
 Flatland
| 3.66 | 207 reads | |
 e
| 4.5 | 2 reads | |
 Zero: The Biography of a Dangerous Idea
| 4 | 2 reads | |
 The Golden Ratio: The Story of Phi, the World's Most Astonishing Number
| 3.5 | 12 reads |