Shopping Cart Cookies
If you buy an eBook the shopping cart uses "cookies" as an essential part of its operation.
Google Advert Cookies
This site contains pages with adverts generated directly by Google. Do NOT click on the advert links unless you are interested in the advertised product. If you do click then a "cookie" may be set on your computer (see below). By clicking on an advert, you agree to Google's use of "cookies" on your computer.
A cookie appears to be written only when the ad converts. When the cookie is not written, the IP number is still recorded. Here's a cookie that was written yesterday for a first-time visit-and-click from an IP that was not yet recorded by Google:
<string of characters>
The cookie expires in one month. You should assume that both the IP number and a unique ID is in the cookie, and that it includes a checksum to detect data tampering.
The code is actually for advertisers who are using conversion tracking.
This is an easy way for advertisers to know if their traffic is converting to a sale (they can put special code on their order confirmation page, or whichever page they would like to know if their clickthroughs are reaching). Yes, a cookie is used.
The cookie that Google adds to a user's computer when he/she clicks on an ad expires in 30 days. This measure, and the fact that Google uses separate servers for conversion tracking and search results, protects the user's privacy.
Users who don't wish to participate in tracking activities can easily not accept this cookie by setting their Internet browser user preferences. These users will simply not be included in your conversion tracking statistics.