1. Dropping Caldera for Mandrake would be a choice I would leave up to you. Most people that start out with Linux try one distro, get it down then move on to try another for different reasons, maybe see the difference.... etc.
2. The difference mainly between distro's is usually just applications, support given if bought and a few smaller details where files reside... etc. If your using KDE.. usually they are all the same mostly, you can install and configure KDE however you want.
3. This doesn't look like a question but that's great you got your internet connection working, most don't get that down ever and give up.
4. There are many books out there, depending on what distro you using, you might want to go with a specific book on your distro as it will explain things in detail and things won't be off so much from a gerneral linux book. If you do get just a general linux book, i recommend Linux Unleashed.
5. Well, there are many programming languages out there, it would really depend on what you want to do or most interested in. If you surf the web mostly and email... you might want to learn languages pertaining to web design... I don't see the web going away anytime soon and don't see that as a waste of time. Good languages to learn for that within Linux would be Perl, PHP, HTML...and so on....