Kobo is going to have an ebook sale tomorrow, and you can find their sales here. The page won't be live until midnight, though.
Barnes & Noble is boasting that they too are having a sale. Buy a Nook online (with a Master Card) and you can get a $20 gift card. You can also still get the $10 gift card when you buy a $75 gift card.
Best Buy is offering a $25 store credit when you spend $250 or more online today or tomorrow and then pick up your order in a store.
Walmart just sent out an email promising deals, but I haven't seen anything to tempt me yet. You can find their deals here.
And of course Amazon has set up a Deals Store for Cyber Monday. As per their usual SOP, that page will feature quantity and time-limited Lightning Deals. There are also supposed to be deals on Kindle ebooks, with some titles available at up to 80% off.