Who: Leonhard Euler.
When: As a highschool student.
Where: On the book "Euler: the master of us all" by William Dunham.
Why: The amount of creativity and genius dispersed among the so-different works of Euler continues to amaze me just now, so it only could have a devastating effect on me 10 years ago. He not only addressed a lot of distinct topics, he layed the foundations of many branches of mathematics and solved with ease many problems that were interesting me at that moment of my life. I learned a lot from him: he really deserves the title of "master of us all".