This should be interesting:
Get 90% off when you refer a friend
Doesn't seem to have a long URL, sorry. The code sent to the "friend" appears to work with any account (tried it with an already registered one). I can't say how the other code will work. I also tried the friend code on three couponable books, and it didn't work on one. Fine print says the code can be applied on select titles but doesn't provide a list.
Every customer who refers someone who makes their first eBook purchase on Kobo will receive a 90% discount code. Discount code will be emailed to the address registered at Kobo, and may only be used once for 90% off a single eBook purchase. Discount code is applicable on select eBook titles only. Discount will be applied at checkout. Offer valid for customers in Canada, United States, United Kingdom, Australia, and New Zealand. Offer valid from March 25, 2014 at 12:00 AM EST through April 13, 2015 at 11:59 PM EST. This offer is not valid in conjunction with any other offer or promotion and cannot be used to adjust amount paid on previous purchases. Promotional code must be entered at time of purchase to qualify for this discount. Discounts cannot be applied nor the discount value refunded once a purchase is complete. Rakuten Kobo Inc. reserves the right to change or cancel this offer at any time without notice.