I loved reading the comments on my Amazon links dilemma post and found them tremendously helpful. If only I could set up similar discussion forums for all the areas of my life where indecisiveness reigns! Especial thanks to Phantom Scribbler and Andy Laties for so clearly defining the terms of the conflict.
So upon careful reflection, it seems to me that there are two reasons to add a link to a book title in a blog post: either you want to give readers more information about the book, or you want them to be able to get their hands on the book easily, or both. I haven't found a more complete or accessible all-on-one-page source of book information than Amazon (though admittedly I haven't looked very hard, and I like Varbidian's suggestion of linking to the publisher's page), so from that standpoint Amazon seems like the way to go, all politics aside.
As to buying the book--well, mostly I get my own reading material from one library or another, and I suspect most of this blog's readers are similarly inclined. But much as I'd like to link to item records on everyone's individual library system so you can all whip out your cards (or just type in your memorized card numbers) and reserve the book, I don't think that would work very well unless you all agree to move to one metropolitan area for my convenience. (Though wouldn't it be great if there were a Booksense equivalent website for libaries? I could link to a composite item record for all libraries in the world, and you could type in your zip code and then just reserve from there...ah, well. Another project for some technological genius somewhere?)
I like Madame Esme's compromise of linking to Amazon for information, and providing a disclaimer and a link to booksense at the bottom of each post. But I suspect I'm too lazy and forgetful to include something like that every time I post about a book.
So here's my own carefully-thought-through, total-copout compromise: I'm going to link wherever I feel like. If I'm just mentioning a book and want to provide a quick link for those who want to know more about it, I'll link to Amazon or maybe the publisher if I have time. If I'm reviewing the book and want to send all dozen or two of you off to read it as fast as possible, I'll probably link to Powell's (which seems to have added a lot of features since last time I checked--they have reviews now. And a Wish List option. So maybe I'll just link to them all the time. But I'm not promising.).
And I'll add a support-independent-bookstores disclaimer and link in the sidebar, where it can compete with the chickens and booklists and archives for rare and precious eyeball attention.
There! Inconsistent, wishy-washy, and unclear! I feel better already.