If you want my oppinion, you can find whatever you want - if not more - online.. You already have Slackware's book on their site (which you can also download and have a copy of it) which talks alot about linux, as well as Slackware's distribution.
IMHO, there isn't much you could learn about 'Linux' itself, except its history, file structure, relation to Unix, and a starter on how to use it.
This is usually explainied in the beginning chapter(s) of any Distribution's book, and then you move on to standard linux usage, like how to change directories, read files, install things, etc.. and then to the specifics of a distro, like how it installs things in a different way than ./config, make, make install, or how the distro arranges its files...
You have
Slackware's book on their site,
Simply Linux (
http://jetblackz.nabaza.org/), very helpfull.
The Linux Tutorial (
http://www.linux-tutorial.info/)
I can recommend some more for you, and most of them you can get a hard copy, however they're also free online (and to download) =]
Whats different in Slackware?
They say slackware is the most 'downright' distro.. It's very close to how Unix works, and some might tell you its a hard distro, but i disagree .
I'ts not as easy as Mandrake or Suse, with regards to installation, what you get the moment after you install (looks, etc...), but its not really that hard too.. And you actually learn ALOT on how Linux works, operates, etc.. with Slackware rather than if you tried Mandrake, or Redhat.
Slackware simplifies installing software. Usually if you want to install a program, you have to download the software, then
./compile
make
make install
Slackwrae has already compiled packages called .tgz packages, whereyou just say
installpkg bla.tgz
and the package manager installes it for you.
Other things are swaret or Kswaret - graphical - (its like windowsupdate), etc... Its really these small things here and there that make it a nice distro, and once you try it, you'll love it =] and you have the backing of all of us here incase you get any problems.
Oh.. Slack is very stable too