On Monday 27 November, all purchases of the PDFs via this site can be had with a 20% discount. Use the code CYBERMONDAY2017 at checkout. Note: this code is only valid on Monday 27 November from 00:00 CET until 23:59 PST (so about 33h hours).
P.S. If you have read and like the book: write a nice review, e.g. on an Amazon store (where everybody goes for reviews these days).