I think it's nice when it's done, as long as it's done respectfully. If the reviewer didn't like an author's book, the author will not gain any points by being critical of the reviewer. Graciousness that someone read an author's book, whether that someone liked the book or not, is key.
I have authors leave me comments pretty often. Others email privately. Absolutely, it can and should be done as long as, like Trish said, it's done respectfully. I gave a bad review of an author's book and his comment was so nice and complimentary that I'm inclined to read his other books, despite the fact that he knows I didn't like his first. Done wrong, it can really hurt the author.
I have had authors leave a comment on my blog and/or e-mail me and the exchanges so far have been pleasant even when I didn't like their book. One author asked if they could link to the review on their site so that they could give their readers a variety of opinions. I agree with Natasha and Trish that if done respectfully, it's not a problem and often nice. But when authors are defensive or nasty, it can be bad.
Occassionally, it happens. I have had Dave Freer respond to a review and David Henderson sent me a book to review after commenting on his blog. People do respond to comments. It is a surprise sometimes seeing who might respond. Also, Linnea Sinclair responded to a review I wrote. It is rather interesting.
I actually had Daniel Tomasulo comment on my blog. It was on a post that just mentioned that I was interested in the book. The comment was so adorable and showed that he really cares about his readers which makes me want to read his book even more.
I've only had one author make a comment on one of my blogs..and it was appreciative.
On the flip side - I think as bloggers we should also be respectful of the authors. While we may not like a book - we should not simply bash without reason. Sometimes there is no reason why I didn't like a book - for whatever reason it wasn't what I was expecting...it doesn't mean others won't enjoy it.
This is an interesting array of thoughts... I rarely respond to reviews, but it is mostly out of respect for readers' opinions. I have heard that some readers don't like the idea of the author 'looking over their shoulder' so to speak. But if reviewers don't mind, nor do readers, I'll probably do it more often!
I have had this happen on my blog and via e mail.The one time it was quite embarassing as I did a post saying I had ordered a book by Kathryn Casey and had made a "friend" request of her on another site but she has not yet responded probably because she had worked out that I was a "stalker" and she responded to my post by apologising that she had not accepted my "friend" request because she was still learning about the internet and could not work out how to accept.And she came back to thank me on my review for her book when I did post that review.
I made the decision not to make bad reviews.If I dont like a book I dont do a review on it.I am going by "if I have nothing nice to say then I say nothing".Basically my loyal readers have been able to work out though which ones are not that good as I always post on receipt of new books and then if there is no review within a month then they know that I didnt enjoy it.
I have found out that most writers do "google"alerts on their names or titles of their books and that's how they keep track of any reviews on the web.