I just finished reading the fifth and last of Pratchett’s and Baxter’s science fiction series collaboration—The Long Earth, The Long War, The Long Mars, The Long Utopia, and The Long Cosmos. Sadly, Terry Pratchett died before the last book was published.

“Long Earth” is the name given to an infinite series of parallel worlds, which can be reached by a “stepper.” Each Earth is slightly different, and if you step far enough down the line, the differences become extreme—different flora and fauna, different atmosphere, gravity, even a gap where the earth no longer exists. But homo sapiens only exist (before stepping) on Datum Earth, the name given to our original earth.

This is a saga with a varied cast of characters. Joshua Valienté is a natural stepper who doesn’t need a device to step between worlds. Lobsang is an artificial intelligence who travels the Long Earth with Joshua. Sally Linsay is a natural stepper and the daughter of the man who invented the “stepper” box. Sister Agnes is a nun who was Joshua’s caregiver at the orphanage. Then there are the trolls, a hominid species with natural stepping abilities who left Datum Earth long ago. The “Beagles” are a canine intelligent species inhabiting Long Earth. There are more characters and groups of characters, but I won’t try to list them all.

Each book has more than one story line and plenty of imagination. You can read any of the series as a stand-alone, but if you want the whole picture, you will want to read them in order.