OrmIrian, there's reading and reading, though.
I do think (well, my experience anyway) that there is a stage where a child can read almost anything in terms of decoding but is still too young cognitively and emotionally to appreciate the subtleties of some more complex books. And then you run the risk that they will skip through it without really understanding it, think, 'well that was a bit pointless and boring' and then you've ruined a good book that they would have enjoyed a couple of years later. That's all.
DD1 read the His Dark Materials trilogy at 7 or 8, for example, and it was definitely too early. Whereas Just William, which is also relatively complex in terms of language has nothing like the same level of conceptual complexity, so is more suitable for a quite young child as it doesn't have that whole extra layer of allusion, symbolism and atmosphere that would just pass a 6yo by. The horror of Lyra being separated from her daemon, for example, relies on the reader having access to a range of emotional experiences and symbolism that is going to be beyond even the most fluent 6yo reader.
Books I would have similar reservations about for a 6yo would include things like:
The Hobbit
Anne of Green Gables (full of teenage angst, wasted on a 6yo)
Little Women (ditto)
The Children of Green Knowe (has a delicately spooky elegiac quality which you need a certain level of maturity to appreciate)
I Am David (ditto, both my older dc read it far too young, which is a shame)
The Secret Garden and A Little Princess would be good to read aloud to a sophisticated 6yo I think, less suitable for them to read to themselves (subtlety again, also the period detail would be lost if read alone by a child that age). The same with something like Ballet Shoes. Also some of the E. Nesbit stories like The Treasure Seekers and Story of the Amulet. And CS Lewis too, I think a child that age would get more out of having them read aloud than reading to herself.
Other ones she probably would enjoy for herself (trying not to replicate other suggestions) are:
Finn Family Moomintroll
The Bullerby Children by Astrid Lindgren
Dick King-Smith
There must be others but my brain is dead atm.