In Boy's Life, what is the most important lesson that Cory learns in his maturation process?
Without a doubt, the most important lesson Cory learns throughout his maturation is learning how to let go. This is very easily demonstrated in his different responses to death in Book 3, Chapter 5, "Case #3432," in comparison with his response to death in "Solitary Traveler" in Book 4, Chapter 1.
In Book 3, Chapter 5 ("Case #3432), Cory prays his dog Rebel back from the brink of death, only to watch Rebel suffer as he fights to stay alive: "He never ate. Never drank a drop. He stayed in his pen, because he could hardly walk on his withered leg. I could count his ribs, and through his papery skin you could see their broken edges" (McCammon 357). Cory's prayers to save Rebel have not saved him the way he most likely intended, and have not restored him to the loving and active dog that he once knew. He sees the quality of life Rebel experiences and finally lets him go, allowing him to transition over to the Otherworld—thus giving Carl Bellwood the dog he so desperately wanted (McCammon 362).
Conversely, in "Solitary Traveler," Cory clearly mentions the lesson he has learned as he refuses to repeat the same selfish request of Davy Ray as he asked of Rebel: "I knew the power of prayer, but I was through being selfish. I wanted Davy Rayy to be all right, of course, and that's what I prayed for with all my heart, but I would never dream of wishing Rebel's death-in-life on a force of nature like Davy Ray" (McCammon 447). Here, Cory clearly and concretely explains the lesson he has learned and how it is now informing his other decision-making.
People may argue that he also learned he cannot "shove a broomstick down [life's] throat," but Cory never outright acknowledges learning that as he does with Rebel and Davy Ray's passing (McCammon 474). In addition, another option is his appreciation of magic within the world--as he discusses in the Prologue--due to the fact that he then writes a book about capturing the magic of childhood, (but again, this is a bit more concrete and powerful).