I run a blog where I analyze books from a writing perspective to find a lesson, of sorts, for other writers. Mostly the posts show what a book did right, but occasionally they highlight where (I thought) the book went wrong. How careful do I need to be about highlighting negative aspects? I'm passionate about what I read, good or bad, and that (should) show in my writing. But I don't want to alienate a potential agent if I disliked a book they repped- especially since the main point of my posts isn't to review a story, but to learn from it.You're right to know this is squishy territory. I am very fond of my clients, and the books they write. However, I do not confuse that fondness with the idea that all the books they write are perfect.
A judicious post, pointing out what worked or didn't, is generally safe ground.
What ISN'T safe is drawing any kind of conclusion about how the book got that way. To wit "the author phoned it in" "the editor was asleep at the wheel" "the agent lost her mind when she signed this one."
You have no way of knowing what went on behind the scenes creatively or editorially.
Focusing on the book is your best plan.
You should also remember that if I love your work, and sign you as a client, all my OTHER clients will be skulking around your blog to learn about you. A lot of my clients are in a mutual admiration society, which I strongly encourage.
What that means for you is: Make sure the author of the book you're talking about will recognize it as a thoughtful, well-written piece, not some sort of hatchet job (at least after the first read!)
What you're also not going to do -- EVER -- is tweet or link to the author or editor or agent about this review.
It's one thing to know there are critical reviews out there; it's another thing to have someone put a reference to it in your timeline.
Your very hesitation on this tells me you'll err on the correct side of caution.
And remember; all the books I sell are AMAZING!