The main feature of this level of books is that each book has chapters.   This represents quite a jump from the previous level where each book contained a complete story in one long chapter.  A feature of 'chapter books' is that they often come in a series, so that once the reader is hooked on the first book they will return to read the whole series.   To get them started on these it might help to share the reading load by taking it in turns to read a chapter each, or just read the first chapter or two to get them interested.