What order should you read the Rucksack Universe in?
Short answer: Whatever order you want.
2 common approaches
The Rucksack Universe is an ongoing, nonsequential series, set in one big story world with many recurring characters and various story arcs. You can come to the Rucksack Universe from any title and enjoy the stories in whatever order you want. Here are 2 ways readers typically approach the Rucksack Universe series:

The order in which each title fits on the series timeline, regardless of release date. Story order is subject to change. Follow along by order of when each story happens within the series.