ColinsMum--well, with these books, I do know other kids who've read them and liked them so my words would be based on more than just my DD (who hasn't read them, actually!) I guess the example is a somewhat inapplicable one because I know both series. I would be more careful with books I didn't know--for instance, DD is reading Inkheart, which might also be good for a kid who likes HP, but I can't recommend it because I haven't read it and really can't comment more than vaguely on it.
FWIW, this all pretty much happened to me last summer, when DD was really crazy about a series and talked about it endlessly such that I mentioned it on FB when she wrote fanmail to the author. Some friends with kids the same age went out and checked out the book, thinking DD's recommendation was worth following. I then got comments on "Uh...my kids cannot possibly read that book. Now I have to read it aloud to her." So...once bitten, twice shy.
