I had- well, still have, to be honest- a number of these books from my childhood though they are considerably battered now, not only from my use and that of various siblings, but more recently from a new generation of nephews and nieces.
As mentioned, these stories are narrated by young John D. mostly about his older brother Tom who, possessing a Great Brain, at various times manages to hoodwink, outsmart, and even swindle not only his naive brother, but the rest of his family, other children in the town, and most of the townspeople as well. He occasionally also uses his powers for good instead of evil, but generally has an ulterior motive for doing so. The books are really funny, and also very relatable to anyone who grew up with an older sibling- or siblings- who delighted in putting one over on you. I speak from experience, being one of nine children. That being said, the books also deal with some very serious- even heavy- topics, albeit from a child's view and understanding- death, sickness, bullying, pain and loss. One story of this sort which particularly sticks in my memory from The Great Brain is that of Abie, the only Jewish man who lives in their little town, which is majority Mormon, with some Protestants and Catholics (the Fitzgeralds are Catholic). Though well liked enough- he runs a small shop- no one really knows him well. They are vaguely aware that his shop isn't doing great now that there are a lot of competitors in town, but no one really worries, because everyone knows that Jews have money; it's been rumoured for years that Abie has a strongbox full of gold coins. So no one raises the alarm when the elderly man steadily becomes frailer, even falling down in the street on a couple of occasions. Then one day he doesn't open his shop, and the concerned townspeople enter his living quarters to find him dead in his bed. To their horror, the doctor tells them that Abie starved to death; he was too proud to ask for help though his neighbours- of all religious stripes- would gladly have given it, and they never thought to ask because, subconsciously, it never occurred to them that a Jew could be poor.