Amazon.com has a special importance if you're an author or editor. Once the cover of your book is set, check with your publisher to make sure the book will be listed on Amazon. This is pretty much a given if your book is being produced by a large publishing house. If your publisher is a small specialty press, a little (or a lot) of wheedling may be necessary. But if you've got more than 300 books to sell after pre-orders have been accounted for, it's best to get the book listed on Amazon.
I have a bit of a love-hate relationship with Amazon.com; some of you may have a hate-hate relationship with them. If so, I sympathize completely. Amazon demands a 55% commission on top of account setup fees*, and they've been bullying some POD publishers into using their Booksurge service instead of LSI and other printers. Amazon is the 80,000-pound gorilla of book sales, and they've been taking full advantage of their status, often to the detriment of small publishing companies.
So, I understand a small-press publisher's desire to tell Amazon to go blow; the publisher's got their own site and can sell books through their own secure shopping cart just fine, so distribution's covered, right?
The problem is, for many prospective readers, if your book isn't on Amazon, it's as if it just doesn't exist. Your book's being available at the publisher's site won't help if a reader has never heard of the publisher before and is therefore reluctant to release their credit card info to them.
So: if your book's not on Amazon, you will lose potential sales. Also, because so many other sites grab book information directly from Amazon's feeds, your book's absence from that site means it will also be absent from (or difficult to add to) a bunch of other sites like LibraryThing and BookMooch.
And finally, because book information posted on Amazon gets distributed far and wide, double-check that the publisher is posting accurate, complete information about your book from the start. The publisher can make changes later, but I've noticed changes often don't propagate to Amazon.uk and other sites. It's better if the book description is correct from the beginning.
* 50-55% commissions are pretty standard for big book chains like Barnes and Noble as well as Amazon; indie book stores generally charge lower commissions.