The excellent Mastering Emacs is 50% off.
If you've been holding off buying Mastering #Emacs then grab a bargain! It's 50% off – just $20.https://t.co/Fg618KEiOA
— Mickey Petersen (@mickeynp) November 25, 2016
If you use Emacs, you really want this book. It’s been updated for Emacs 25 and is only $20 dollars while on sale.
UPDATE : matering → mastering