[1] It consists of 32 books, each 32 pages long and illustrated by Pat Cook (1974-1979) and later Ray Mutimer (1980), written in a simple vocabulary.

[1][2] Four paperback "novella" books intended for older readers were also published in 1983 by Arnold-Wheaton.

The books begin with Tim finding a key which enables him to see the Hidden People, he befriends Tobias the black cat and has many adventures.

[7] The books were cited as an inspiration by the author Victoria Biggs, who used the "Hidden People" as an analogy for those suffering from dyspraxia.

[9] Series A to D were reprinted in single novel form, with reduced quantity black and white images, but with expanded text.