Chances are the author doesn't understand the issues. To keep the system going they will have to boost the reaction mass to match orbit with the spacecraft.
Recently read a science fiction book which described a similar concept, but I forget what the drive system was. May have been solar powered ion drive. That author called them cyclers with three cyclers continually cycling between Earth and Mars.
Done right, once the cycle is going the system won't require a lot of additional delta v. Shuttle craft would be required to boost cargo and consumables to matching orbits, but the cyclers themselves would be very efficient.
Read that again, carefully....
To refuel and resupply the cyclers, you would need to inject your shuttle craft into an interplanetary trajectory.
Doesn't sound "very efficient" to me.