How did Dr. Bell help Helen? Why did she dedicate her book to Dr. Bell?

Alexander Graham Bell helped Helen by directing her father to the school that might best be able to provide Helen with a teacher, and indeed they did. Helen dedicated the story of her life to Bell because of his work with the deaf and because he was the first to start her on her journey toward communicating and learning.

 To
ALEXANDER GRAHAM BELL

WHO has taught the deaf to speak
and enabled the listening ear to hear
speech from the Atlantic to the Rockies,
I Dedicate
this Story of My Life.

Helen's parents began their quest for a teacher for Helen when she was "about six" by going to Baltimore to consult the "eminent oculist" Dr. Chisholm. Although he could not help Helen's vision, he referred the family to Dr. Alexander Graham Bell in Washington, D.C. as the one most likely to lead them to who could educate young Helen. Happy to have them come to him in Washington, Bell referred the Kellers to Mr. Anagnos, who was "director of the Perkins Institute in Boston."

Helen's parents had heard that the then-deceased Dr. Howe of Boston had found a way to teach children who were blind and deaf. The Perkins Institute is where Dr. Howe did his groundbreaking work in teaching techniques. Helen's father followed Bell's recommendation immediately, and "in a few weeks there came a kind letter from Mr. Anagnos with the comforting assurance that a teacher had been found." Dr. Bell helped Helen by leading her to the source that would provide her educator—Anne Sullivan—and her education, and Anne Sullivan became a loyal friend.

According to the wording of the dedication, Bell "has taught the deaf to speak / and enabled the listening ear to hear." Bell invented the telephone so that anyone listening might hear speech from anyone else "from the Atlantic to the Rockies." The first reason refers to Bell's work with teaching deaf students "oralism," or the ability to vocalize speech sounds even though they couldn't hear speech sounds. Originally based on his father's "universal alphabet," but later modified to simpler techniques, Bell taught what his father called "Visible Speech" and which Bell later, after he'd made modifications, called oralism. This work and the invention of the telephone are the two reasons Helen states for her dedication. Underlying these reasons was the deep friendship Helen shared with Bell.

I at once felt the tenderness and sympathy. . . [that] would be the door through which I should pass from darkness into light, from isolation to friendship, companionship, knowledge, love.

Dr. Bell went everywhere [at the 1893 World’s Fair] with us and in his own delightful way described to me the objects of greatest interest. . . [H]e made me understand how it is possible to send a message on wires that mock space and outrun time, and, like Prometheus, to draw fire from the sky.