I hope they've controlled it for an increasing proportion of the later books being aimed at Young Adult audiences...
(Though I guess that cuts both ways and you could argue that writing more simply using the established parts of your legendarium is exactly what you might aim for if you were less consistently able to handle complex plot threads and novel worldbuilding)
Yes, they did.
> Eight titles were excluded from the analysis due to them being either shorter than the other full-length novels (Eric, 1990; The Last Hero, 2001), or because they are part of his titles for younger readers (The Amazing Maurice and His Educated Rodents, 2001; The Wee Free Men, 2003; A Hat Full of Sky, 2004; Wintersmith, 2006; I Shall Wear Midnight, 2010; The Shepherds Crown, 2015).