The room was just as messy as it had been all week; the only change was that Hermione was now sitting in the far corner, her fluffy ginger cat, Crookshanks, at her feet, sorting books, some of which Harry recognized as his own, into two enormous piles.Crookshanks is said to be part kneazle, which might explain why he's intelligent and hard-working enough to sort books like this.
Even before that ambiguous modifier, the sentence's fourth word "just" had also appeared in the previous sentence with a slightly different meaning, producing a bit of verbal discord. And anytime a modern novelist sees a semi-colon in a manuscript, that should be a warning to consider breaking up the sentence instead.
Which might produce something like this:
The room was no cleaner than before. But now Hermione was sitting in the far corner with her fluffy ginger cat, Crookshanks, at her feet. She was sorting books, some of which Harry recognized as his own, into two enormous piles.And you know what? The second version's six words, or over 12%, shorter. In a book over 700 pages long with over ten million copies in print, that can add up.