I first read The Great Brain series in grade school. I recently came across a few of them and used Amazon to track down the rest, since books 4, 5, 6, and 7 are out of print. The Return of the Great Brain, Book 6 in the series, picks up in the wake of Tom's reformation at the end of book 5. We watch as Tom has to take his scams to the next level so the entire town of kids doesn't realize they are still being swindled. Will he pull it off? Read it and find out!
An adult can read each book in this series in perhaps an hour or two, since they are such easy reads and work as chapter-books for youth. The stories are more well-crafted than I originally gave them credit for being. Fitzgerald really pulled it off: The perspective and subtext of the stories changes entirely for an adult reader, who can see shades of meaning that the kids will miss, without missing the fun and adventure that the kids love most. Fitzgerald's verisimilitude and rock-solid structure make this possible, while his sense of life and authenticity makes it a pleasant read from cover to cover.
It's a shame the only books currently in print (as of this writing) are The Great Brain (1), More Adventures of (2), Me and My Little Brain (3), and The Great Brain is Back (8), which was a posthumous epilogue put together from the author's notes after his death. The four books that are out of print are easily the best four in the series: The Great Brain at the Academy (4), The Great Brain Reforms (5), Return of (6), and The Great Brain Does it Again (7). Fortunately, there seem to be a good number of these in circulation. Grab them while you can, and hopefully some publisher settles up with the Fitzgerald estate so the rest can survive for the next generation of kids.