Slackware package updating
Think of as an equivalent of the Arch linux's AUR or the Ports system for Free BSD.The official method to install a Slackbuild is to do so manual per the instructions on the site there is however a great tool called sbopkg which provides similar features to what slackpkg does only specifically for the slackbuilds repos.There is a repo but it's rather small, slackpkg pulls from it.If you want packages that aren't those repos of course you'll have to get them elsewhere.The less you install, the less user friendly it gets.
However there are quite a bit of third party applications for installing them.
Alien Bob (Eric Hameleers) and Robby Workman are part of the Slackware development team and are extremely respected members of the Slackware community.
There are other random package sites for Slackware but none of them are highly thought of as those mentioned above.
It provides an ncurses interface with which to browse and install Slack Builds.
For documentation, take a look at the Slack book, it's got a lot of great information. Really, to learn about Slackware the best advice I can give is to use it, visit the forums at Linux Questions, and read the Slackbook. I've long heard that this is the distro to use if you want to learn Linux.
Search for slackware package updating:
As for documentation the official set is included right in your DVD medium and housed on the official repos.