I found on Amazon one of my images on a small travel/local interest book cover that has been published both as paperback and Kindle ebook.
The cover contains the book title, my image, and smack under the image is the writer's name. As it is customary on Amazon, you can display the "Look Inside The Book" section that shows the book cover in full size (in this case without any copyright notice).
 
I don't know if the image acknowledgment / copyright notice is included inside the book or not, but the present book cover design would imply that the image was created by the person whose name is underneath the image.

Is it legal not to include imagemaker's name and copyright notice on the cover?
 
Here is the link to the book:
http://www.amazon.com/Moon-Spotlight-Georgian-Cottage-Country-ebook/dp/B00AUZSA0W/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1392449770&sr=1-1&keywords=georgian+bay#reader_B00AUZSA0W

I think -- depending on the exact license bought -- this is very likely legal. Nobody would expect that the name on the cover of a book is the name of the photographer of the cover image. It's clearly the book's author's name. And you as the photographer were probably credited inside the book, as is customary.

I presume the image was purchased from a stock agency, so I was questioning only the absence of the credit notice. It seems that the only way to find out whether there is a proper photo credit is to purchase her book.

Coincidentally, the same image was published last year also in Ontario Travel Guide 2013, and they did include both Shuterstock and my name in the left bottom corner. I think if the images are displayed in a digital publication, the photo credit should be shown right in the image (especially in book previews that do not show inside pages or page with photo credits).
 
http://canadatravelguide.advanced-pub.com/Vizion5/viewer.aspx?issueID=1&pageID=112

In addition, I display the same scenery in panoramic format as the top image on my blogsite, so the image has been around:
http://www.advantica.wordpress.com

Sometimes checking 'my name/istockphoto' or 'username/istockphoto' (or iStock sometimes) takes me to a credits page on Amazon or GoogleBooks (I think it's called?), though I don't get to see the page my photo is used on.

You have two separate issues - whether the image was legally purchased and whether it should have had a credit.

If the image has had several sales, it could be virtually impossible for you to know whether it was legally purchased. It's the clear downside of the RF model, and astonishing that the RF inventor didn't realise this in advance, or at least once the internet became well established. In hindsight (tell me about it!), it could be better for you to put some sort of watermark on any images you use on your blog. On my Flickr stream I have the title and my copyright on every image, though mostly non-stock. On my (unloved) blog and Facebook page I put a title on the photos, even though the latter are mostly 'happy snaps'.

Assuming it was legally bought, you then have to know where it was purchased from. If it has sold from several agencies, you need to read the licensing conditions for each agency to see if they require a credit in this instance. I found a biography whose cover shows the subject superimposed against one of my photos. I bought it mainly because it's the sort of book I have a shelf full of anyway, and also I thought the 'bragging rights' would be fun. Anyway, having bought it, I discovered no photo credit, neither for my pic, nor for the photo of he biography's subject. I questioned this on the iS forum (hahaha back in the day!) and a few people said, more or less, 'it's a courtesy, it's not required', and no-one disagreed.

I have to also say that iS does require in its licensing agreement that editorial images are credited username/iStockphoto, but at least 2/3 of the editorial in-uses I've found via GIS have no credit.

I would say a book cover is more promotional, than editorial, irregardless of the content, so wouldn't require a credit on or off the cover.  Although that may be publishing tradition, in some form, it isn't a requirement.

Thank you, Sean and Martha, that's good to know.