
There is no limit to how many times you can use it and if the book you want is under three dollars, it ends up being free. For example, if you search for Samhain books
You can search for any publisher name in the search box at Kobo, although you don't always get only books by that publisher, then sort by price. You may also want to check out their Free eBook List
If you find a book that is more than $12, you might want to try the coupon code Save25Kobo, which works with some of the NYT bestsellers (thru 12/15). If you are a true night owl, you can pick up Bill Bryson's A Short History Of Nearly Everything for $8.89 tonight, from 10PM to 8AM. I don't know what happens at 10PM, but right now you can use the $3 off coupon code, so I suspect when the price drops later tonight, that will still work, yielding an effective price of $5.89.
Note that all Kobo books are in EPUB format and have DRM added, even the Samhain titles that are generally DRM-free at B&N. What that means is that you can't read these on your Kindle, but they can be read on the Sony, nook and Kobo ereaders or on your PC, Mac or iPad/iPhone using one of several different apps.
Important Tip: Be sure to click on Apply after entering your promo code and verify that the amount shown above the Buy Now button is $0.00, before clicking!
The only snag I've run into at Kobo is that, due to their being a Canadian company, some US credit card companies may not let their charges go thru. You may have to contact them to get them to authorize the charges or use another card. Also, some banks and credit card companies (although not all) may charge you a money exchange fee. So far, you can't set up a gift certificate there, so you may want to just try a few charges and see how it goes. The free books, whether free before or after the coupon code, will only require the account be set up and there shouldn't be any actual activity on your card.