This is a big one, many successful affiliates have popular newsletters and opt-in lists. This has gotten a bad name due to the mid-late 90's when spammers realized this was a possible source of income and began to send massive amount of unsolicited email. This is not only condoned within the affiliate marketing community, it is illegal in most countries. To legally send mail, you must get an “opt-in” as in the customer requests to be added to the mailing list, which is where ebooks come in handy. You may very well have gotten this guide as a gift for joining my mailing list or as a gift to join somebody else's mailing list.
To create a successful mailing list you will need an autoresponder service, by a wide margin the most popular is Aweber. It offers a variety of pre-made templates and requires an industry gold standard double opt-in for mailings to be sent. To get subscribers you should have some sort of gift (you can use this book if you'd like) that they will be given should they complete the double opt-in process. Then I would recommend either a weekly newsletter or a monthly sales letter describing new products. Manage this list well and you will see great profits, do not be afraid to push a good product but if you push bad products to make a profit your list will evaporate quickly.
The best way to find new subscribers to your list is through promotions on your websites as well as putting it in your author resource box for articles that you write.