The referenced deal has expired. This post is kept for historical records only.
HumbleBundle has a great deal on a bunch of technical publications related to Linux and Unix from APress. Here's a list of all the titles available on this deal, with links to their Amazon counterparts. The higher tiers include titles that cover relatively advanced and somewhat obscure subjects, but the lower ones are well worth the few dollars they're asking for.
APress is a reputable publisher, barely a notch below O'Reilly; however, I recommend that you read the reviews on Amazon (linked below) before spending too much on documents you may never get to or through.
The deal ends on January 18, 2021.
$8 (all the above + ...)
$15 (all the above + ...)
$18 (all the above + ...)
Note that the hyperlinks above are pointed to the matching Amazon product pages. The URLs have been thoroughly sanitized using this Amazon URL cleaner.