Honestly, the only thing I'm really referring to is Holo's apparent age. The old anime kind of made her plausibly young adult in appearance.
The books are clear that she's supposed to appear as a young teenager, 13-14 iirc.
It's not quite that explicit. Now, this may be a side effect of the English translations, but the book often uses language to describe her appearance as childlike: girl instead of woman, teenager, daughter, etc.
I did go check the books (it helps that they make an effort to describe her anew at the beginning of each one) and it's not as explicit as I remembered, but it's still very potentially problematic.