Great Manual On www.slackware.com
I was a Red Hat Linux user before I got into Slack. I found reading and following the html Slack Book online was my best tool. You might start off with a few difficulties but if you slowly walk through the install process you will be good. Yes, seeing the text as apposed to a nice pretty GUI does catch you off guard, but really don't fret. It walks you through everything!
As for learning, "Practical Guide To Linux", The Slack Book (I purchased it a year ago to finally pay them for their great work on this book), The Linux Bible is ok, and since you are using Slack you might even find some command help in most Unix books. Good luck, enjoy Slack and its great features!