There are three levels of difficulty in these books, with the stories also getting more complex as the series goes on.
Level one, the stories use only 100 words, limiting the range of the plot. The books are one story from book one to six, followed by platform readers for the strugglers in two parts, 6.1 - 6.6, then 6.7 - 6.12. These introduce new characters, Eggin, the good guy and The Buzzit, the bad guy. Other creatures also enter the storyline.
Level two contains a Scottish clan of Fuzzbuzzes living in a glen and the Slinx going into space. Books 7 - 12 follows a simple introduction of the clan in the glen, living in crofts. Stories are entertaining and amusing. Books 12.1 - 12.12 follow the progress of the Slinx versus the Fuzzbuzzes in a territorial battle over the dump. The use of materials recycled from the dump and some of the scientific processes used especially appeal to boys who aren't so keen to read.
Level three is also based in Scotland and the clan venture into the Glen of Gloom which is inhabited by a monster, The Snagron. Vocabulary in these books remains largely phonic, but ventures into words using familiar spelling patterns, so still easier to sound out than normal text. Rich in adjectives and bloodthirsty in places, these stories appeal to the older reader and boost confidence by using text that is dramatic yet logical. Plenty of wonderful illustrations to keep the more reluctant readers gripped by the sequential books numbered 13 to 20.
All of the books have additional tasks at the bottom of some pages, which can be written or completed orally, including close text, ie 'find the missing word'. Many books have comprehension questions at the end of the book giving page numbers where the answer can be found. The story books also come with workbooks for the children to practice writing and comprehension in.