What I've Learned About Links
Most Clicked - The most popular link in a single-offer e-mail is the graphic button-type link that follows a complete description of the offer. Buttons far out-pull text links and they are more likely to be clicked if they are not placed above or to the side of the copy that tells the consumer why they should click. This is not to say that you shouldn't offer other options for clicking, just that the button following the offer should be the most prominent.
Just Browsing - In a shopping e-mail with multiple products, provide browsing categories as well as individual product links. A consumer may not love the particular book or blouse you're showing, but may be interested in browsing "New Mysteries" or "Summer Shirts & Tops." Provide lots of options; you don't know which will attract interest.
Link Everything - Your HTML programmer will not love you for it, but link everything -- the headline/subhead, the image and the text or button -- to the desired Web page. People click things whether they appear to be links or not and you don't want to interfere with that impulse. On that note, be sure to link your logo to your Web site home page, for those who aren't interested in a specific call-to-action in your e-mail, but are motivated to go to your Web site nonetheless.
More Is (Usually) More - More links generally means more response overall. You have to experiment to find the right number for your offer and audience. But beware, consumers have a set attention span and they will allocate it among the options available. If you want your readers to focus on one offer, don't distract them with lots of competing offers and extraneous links.