Humble Book Bundle for Unix

Link has a few Linux books in it (no objections to that), but given that the question is asked in a FreeBSD forum, I would expect to see some FreeBSD related books listed in your url.

No need to explicitly say that book and this book, amazon returns plent of results when "FreeBSD" is used as the search keyword.
The link gives several books of use to FreeBSD administrators. The point the original author is making is that Humblebundle, which, as far as I can see, does some good things and donates money back to various things, is offering some of these books at very nice prices.
Well, no FreeBSD books as such. As scottro mentioned, some of these books could be used by FreeBSD administrators as they are Unix-based. Some of them are also quite old. For instance, 3 of the following 4 books are from 2002, with the last one being from 2006:

Essential System Administration, 3rd Edition
TCP/IP Network Administration, 3rd Edition
Network Troubleshooting Tools
DNS and BIND, 5th Edition

I don't currently have any of these books and was just curious whether any of these books were still worth purchasing. I guess the fact that they are being offered for sale and newer editions don't exist means that they probably are.
I already had some of the books (sed & awk, shell scripting, dns and bind) but got myself the bundle anyways. Even if some are a bit old, most of the topics discussed in them haven't changed that much. E.g. the books on sed & awk or DNS and BIND are still >90% spot-on, with a few minor additions here and there, mainly for advanced topics or edge cases.
Even if I might not touch some (half?) of the books, the ones I'll read are still worth it...

I found over the last few years, that most "new" books on software that has been around for decades, are bloated with lots of chit-chat and lots of repititions and mostly written in a dumbed-down "how-to" style for most of the book, where the author(s) only describe one way of doing it. Maybe with some small variations, but mainly you are only halfway off the ground and start digging the manpages and additional leterature to really get started, because the really interesting/trickier parts are just merely touched or left out completely.
Older books are mostly way better to the point and more generic in that they cover a broader spectrum in a more condensed and precise way. My absolute favourites are still K&Rs "The C Programming Language" and "The AWK Programming language" by the authors of AWK. They are straight to the point and yet easy and somewhat entertaining to read and cover essentially anything you need in around or under 100 Pages, where newer books often offer much less information in 300 pages or more...

Oh and BTW: you can also pick the FreeBSD Foundation as the charity to donate to....
Just in case anyone missed it:

I'd be interested whether there are any books on the list which people feel are no longer relevant or should have been included but weren't.
I am too tired to compile my lists now. You are missing lots of essential UNIX books. At the same time you have several titles about BASH (Broken Again Shell) which is not an UNIX shell. The same goes for Emacs and VIM which are not UNIX tools. UNIX has only three editors: ed, ex, and real vi or nvi. Linux != UNIX. What is that book about Linux doing there?
BSD Unix Tollbox (1000+ Commands for FreeBSD, OpenBSD, and NetBSD) is useful, but it does focus on Bash, and it is insufficient on its coverage of firewalls.

It would have better if that list focused on Korn (mksh, pdksh), and original Bourne shells. Is that list for O'Reilly books only?
I'm going to respectfully disagree with you folks complaining. The OP was kind enough to share a link that has several books useful to BSD admins. Any complaints about the selection should go to Humble Bundle IMHO.
As has been written, No good deed goes unpunished. :) The sed and awk and vi books, for under 10 dollars probably make it worth a mention here.
It's a great bundle of books and well worth the purchase even though I already have several of them in hard copy. Thanks to xavi for the heads-up. The "vi and Vim" or "GNU Emacs" depending on your personal preference, "sed & awk" and "Classic Shell Scripting" are the pick of the bunch in my view.
I'd be interested whether there are any books on the list which [...] should have been included but weren't.
Since knowing how to use a shell properly can really make a difference to one's productivity I would have liked to have seen more than shells/bash covered. "Using csh and tcsh" is a great book, particularly from a FreeBSD perspective since tcsh(1) is included in the base system. I've tried the various popular shells but always come back to tcsh. Properly configured I've found it can just about match most of the functionality that initially led me to try heavier options like shells/zsh.

I really hope O'Reilly finds the bundle sales approach works well for them. It would be great to see other bundles with the O'Reilly development books or application books. This link will get you there... for the next few (edit) hours days at least... with the FreeBSD Foundation selected as a charity though you'll need to change the amounts manually to your satisfaction.
I'm going to respectfully disagree with you folks complaining. The OP was kind enough to share a link that has several books useful to BSD admins.
Ok I have little more stamina so here are few books I recommend so that you see where I am comming from.

Every study of UNIX should start by reading Kernihan and Pike's master peace The Unix Programming Environment also available on-line in PDF form

Once you have read through that book you should sit in front a computer running an old fashion UNIX (OpenBSD comes to my mind) and start playing with examples from the book. The natural progress would be reading man pages. UNIX-es typically come with stellar man pages (Solaris and OpenBSD come to mind). My biggest grouch with novice users comes from the fact that they start surfing web from some random information instead of reading man pages. People read man pages! Learn how to use apropos command

From there one can expend by picking a specialized books which expands on each chapter of the Kernihan's and Pike's book. For example logical expansion for Chapter 3 which talks about shell programming would be reading

UNIX Shell Programming, Revised Edition
Dec 1, 1989
by Stephen G. Kochan and Patrick H. Wood

(stick with older edition to avoid Linux non-sense). Then graduate with something like

The New KornShell Command And Programming Language (2nd Edition)
by Morris I. Bolsky David G. Korn (Author)

Fourth chapter of Rob Pike's book talks about filters. The basic example of a filter is surprisingly editor ed. Your FreeBSD box should just like mine OpenBSD come with

Read ed man pages. Read ex man pages. Read vi man pages. Then get the following second to none book about vi.

Ultimate Guide to the VI and EX Text Editors

OP listed very nice book for sed

sed & awk (Nutshell Handbooks) 1st edition by Dougherty

Again avoid 2nd edition because for Linux-isims. Also don't learn AWK from that book. Get the best programming book every written. The AWK Programming language by Al Aho.
At that point one should realize that studying regular expressions is must.

Mastering Regular Expressions 3rd Edition
by Jeffrey E. F. Friedl (Author) has everything in it from the point of view of a Perl Guru.

I am not sure where I am going with this post as OP wanted essentially advise on the whole UNIX universe. I have to finish somewhere so this is final. If you are system admin no reading is complete without reading one of older non Linux versions of
UNIX and Linux System Administration Handbook
by Evi Nemeth and Garth Snyder

make sure you understand filters and pipes in particular.

If you are kernel hacker Design and Implementation of BSD 4.4 and Bach's
The Design of the UNIX Operating System
Jun 6, 1986
by Maurice J. Bach

are must
Thanks for the great list, Oko.
You are missing lots of essential UNIX books. At the same time you have several titles about BASH [...] Emacs and VIM which are not UNIX tools. [...] Linux != UNIX.
You are right here: the title of the bundle is a bit misleading but I'm happy to ignore that and just consider it as a set of (mostly) useful books. Who knows, maybe the shells/bash books will be useful when I find myself having to debug some script.
My biggest grouch with novice users comes from the fact that they start surfing web from some random information instead of reading man pages.
Man pages are awesome (I'm not talking about on GNU/Linux here, where my experience with man and info pages has been very different from FreeBSD) but in my view not a great starting point for a novice, even if searching with apropos(1). Well-written books that introduce functionality in a logical order are best, but not free to buy nor available everywhere in public libraries. It's perhaps inevitable that new users search the web for something available instantly and at no cost. The FreeBSD handbook is a great place to capture useful introductions. For example, the section on PF is much more accessible than the pf.conf(5) page. I would say the same about the OpenBSD PF User's Guide compared to the OpenBSD pf.conf man page.

I'm not sure the same can be said for the section on text editors. I'll add writing a vi(1) page for the handbook that expands on the concise but minimal "Fast Startup" section of the vi(1) man page to my very long list of things I would like to do...