Lenny's Book of Everything by Karen Foxlee

I knew my brother. I knew when he talked too much about Timothy his imaginary pet eagle. He was scared.
'Whatever you do,' I said to Davey on the walk to school, 'Do not tell people about your eagle. Do not tell Miss Schweitzer about your eagle.'
He looked crestfallen. His shoulders slumped. He looked to make sure Timothy hadn't fallen off.

Lenny, small and sharp, has a younger brother Davey who won't stop growing - and at seven is as tall as a man. Raised by their single mother, who works two jobs and is made almost entirely out of worries, they have food and a roof over their heads, but not much else. 

The bright spot every week is the arrival of the latest issue of Burrell's Build-It-at-Home Encyclopedia. Through the encyclopedia, Lenny and Davey experience the wonders of the world - beetles, birds, quasars, quartz - and dream about a life of freedom and adventure, visiting places like Saskatchewan and Yellowknife, and the gleaming lakes of the Northwest Territories. But as her brother's health deteriorates, Lenny comes to accept the inevitable truth; Davey will never make it to Great Bear Lake. 

An outstanding novel about heartbreak and healing by an award-winning author.