If you have the images (iso files) for the thirty CDs, then you could add them to your sources.list, which will make the programs on them available for you to install. You would need to burn the images to cdroms. Then, having done this, open up a terminal or console (assuming you have x-windows installed -- if not, just use the commandline), and enter the following:
If this doesn't work, then you need to become root. So, assuming you know your root password, enter the following:
It will then ask for a password. The sequence will look something like this:
Note, the dollar sign indicates a regular user, whereas the hash mark indicates the root user. Now, enter the "apt-cdrom add" command, and it will tell you what to do. This will add all of your cdroms to your sources.list. At this point, you can install programs from them with synaptic or apt-get or aptitude.
Of course, as others have already said, if you have a high speed internet connection, it may be best to just rely on the internet for getting and installing programs. So, try the following (either prefacing the commands with "sudo", or if that doesn't work, then as root, as described above):
This will open up an editor in your terminal. Look and see if you have any repositories there, which will look like the following:
deb cdrom:[Debian GNU/Linux 5.0.0 _Lenny_ - Official i386 DVD Binary-1 20090214-16:54]/ lenny contrib main
deb http://debian.yorku.ca/debian/ lenny main contrib non-free
deb http://www.debian-multimedia.org stable main non-free
The first one is a cdrom repository, while the second one is an internet repository, and the third one is also an internet repository for multimedia programs. "repository" in this setting means, basically, a collection of programs. If you don't have any internet repositories there, then feel free to add the internet repositories that are listed above. Afterward, exit and save your sources.list file by pressing Ctrl-X (to exit from nano). Now try the following:
At this point, you should be ready to install programs. Still in the terminal, enter the command "aptitude" (as root or prefaced by "sudo") and you will see an ncurses dialogue that list programs available for install. Ctrl-T gives you the menu. The forward slash (/) key is the key for searching for programs that you wish to install. Try finding the program openoffice.org and installing it. You'll be surprised at how easy it is to install.
You could use synaptic rather than aptitude, but I personally think synaptic is shit; so, I generally don't recommend it. However, if you must have a gui, then by all means, use synaptic.
I've never actually set up an external drive as a repository for installing packages, but apparently it can be done. See this site
and this site
for more information.