I have to admit, though, I am worried about DS following in my footsteps and becomming overly dependant on extensive series. I didn't really get into reading one-off fiction until I was 14, and really, that was NOT ok. I was genuinely scared of unfamilliar characters/settings. DS seems to be showing some similar tendencies.
My DS7 started at 5 refusing to read any chapter book that didn't have exactly 10 chapters. After about a year of that he started reading books with more or fewer chapters, but that progressed to the desire to read every book in a series. He would change authors and series, but only after he had read every book in the series. Right now he's on Hardy Boys, which should keep him busy for awhile since there are 100+ of those! I think, for him, it has to do with his OCD nature to be thorough and not miss anything. I know I have a similar OCD thing where I have to read literally every word of a magazine before I can recycle it for fear that I miss something, or I have to reread passages in books if I feel like I wasn't really focusing completely on what the book was saying. Obviously I have issues
but I think DS has the same need to be thorough.
