By R. A. Montgomery
Your child pilots a one-person sub down toward Atlantis, choosing which way to go at every turn, and a good number of those forty-two endings drown them in a single paragraph.
Your child climbs into a one-person submarine called the Seeker, the hatch seals, the light goes green, and they head straight down to look for Atlantis. Montgomery makes that whole pitch in about a page. After that the questions start coming out of the dark. Follow the glow ahead or steer away from it. Surface while there's still time or push deeper. Trust the people your child runs into down there or get clear of them fast. Forty-two endings wait at the bottom of all that choosing. A few of them reach the lost city. Others leave the Seeker out of air at a depth where that is the end of it, spelled out in one short paragraph, with the book telling your child to flip back and try the other way. This was the first one Montgomery wrote, before the series even had a name.
The sea is doing something here that a cave or a time machine can't. Your child already knows, in their gut, that sitting at the bottom of the ocean inside a small metal shell is frightening, so this book has tension on page one without spending a single word building it. That is a real gift for a child who won't sit through a slow start. It's the shortest of the old three, barely over a hundred pages, and it's written as 'you,' so there's no hero to keep track of and nothing to learn before the good part starts. And forty-two endings means the child who reaches one hasn't really finished. They go back to see what was down the branch they skipped, and that second trip is more reading they never notice doing.