Forum Moderators: skibum
I used to provide links to Amazon, BN, Powells, and even BookSense for some of the books on my site, though I didn't bother to do so for every book. But I dropped the links to all the sites other than Amazon, because I got VERY few sales compared to Amazon sales, and so far as I could tell, I actually LOST sales when people clicked through to, say, BN, didn't buy anything, then didn't return to my site.
After I dropped those links, my Amazon sales went up significantly, more than making up for the small income I had been getting from the other bookstore sites.
Your site's needs and demographics may be different, but given the name recognition Amazon has, and the high level of trust people have in it, I think most book-recommendation sites do best with Amazon alone.