Settle in with a box of tissues for this one. It's a gut-wrenching read.
Lenora Spink struggles along with her mother when her father walks out to go to work one day and never returns; he's found a job far away from his family, like he's done many times before, but this time, a few years after his son Davey is born, he never returns. It devastates her and her mother, and they struggle on.
Then her little brother starts growing...and just never stops. By the time Lenny's in 3rd grade, her little brother is taller than she is.
The neighbors warn her mother to take him to the hospital, get him checked out, but she insists big folk run in her family. A family doctor says his mother is worrying too much.
By the time Davey's been kicked out of several day care and nursery programs for being "too big," they win a free set of Burrell's Build-It-At-Home Encyclopedias. Lenny and Davey read it daily, starting at "A" and working their way through the sets. This is the framework by which the rest of the story unfolds, alphabetically.
When their mother finally takes Davey to another doctor, to get a second opinion, he's almost six and waaay taller and bigger than Lenny. He's as tall as the next door neighbor, Ms. Gaspar, who watches him. He's as tall as a 4th grader by 1st grade.
He's eventually diagnosed as having a form of pituitary gigantism, which no one can properly diagnose or treat.
In the end of the book, the author notes she created a form of gigantism that defied doctors in the 1970s, when the book is set, but that now it can be treated when diagnosed if caught early enough, in many cases.