Yep, can confidently say I'm using answering or call service which continues to give highest ROI, so I'm also promoting with success via mail order...specific magazines.
Remember you gotta test online first and determine the winners before scaling-up.
Most CB products are focused on specific niches, so you need to target audiences.
The products are of course digital downloads and the main advantages are the info is up-to-date, easy & fast, (apposed to waiting for physical book to be shipped).
Here's a simple formula for success...test campaigns online to extract highest ROI, while building your list (use squeeze page to get physical address) send promotional sales info via offline (post physical letters, etc).
You can either send customers back to your website whereby sales are made online or set-up a merchant account and take orders directly over the phone. You'll most likely find most of the profits are made over the phone...people like to call and buy!
Cheers!