No specific phonic structure to these books, as children should be able to work out most reasonably common words without difficulty.
Vocab choices are appropriate to this young readership and not too complex or demanding
More scope for complex inference creeping in – e.g. sometimes the reader may need to read between the lines, or may know something the characters do not, or the author may set up unusual twist or surprise ending.
Stories are slightly longer and more complex than before and bring in knowledge from outside the story in order to understand it.
As above, though stories are becoming more varied in plot as well as sentence structure, sometimes including some more literary or stylised language where appropriate.