Hey! Those were the two Debian specific books I bought too! Both awesome, took some from one, some from the other, and built more knowledge of Debian goodness.
Between those and the two forums, the official Debian forum and the linuxquestions.org one here, you can learn all you'd ever want to know and more! Don't forget the official Debian Wiki as well. They have a nice NVidia document describing the differences pro and con as far as which way to go, the Debian way or the NVidia script.
The Fedora folks are dead set against using the NVidia script method, as they just don't hold with the way NVidia replaces the official mesa/xorg versions of the openGL plugins with their own.
Myself, I've used only the NVidia script while on Debian as it just looked easier to me because of not having the latest, or any, version available in the Lenny distribution for all the time I used it.
On OpenSUSE 10.2, which I used prior to Debian, I had tried both ways since they also offer a repository. They even make it more simple now with a one-click installer for it on OpenSUSE 10.3. But I usually wound up with the NVidia script as I wanted the newer drivers when they came out, as well as the fact that the Nvidia OpenSUSE 10.2 drivers in that repo were never upgraded to rebuild the module for the Kernel updates that were pushed from Online Update after the first ones. So I was really kind of forced to there, if I wanted NVidia 3D I needed to use the script.
But on 10.3 I found that the repo driver not only was setup to handle Kernel upgrades, but took no action needed by the user to have it automatically rebuild its module whenever a Kernel upgrade was pushed and accepted. Even Xorg upgraded packages were seamlessly dealt with automatically. Really nice!
But I'm on 10.3 now, and you know what? I'm using the NVidia script because first the beta 100.14.23 fixed the Compiz exiting to terminal problem, then the new 169.04 fixed the XVideo color kaleidescope appearence after Compiz had been running for some time. Various other fixes and improvements and even some new bugs!
Yeah, I just like using the script even though it forces me to nvidia-installer --uninstall and then install the driver again after any Xorg or Kernel update.
So you just have to decide it for yourself. Since you already have installed them the Debian way you're all set for now. Remember that you need to run module assistant again if a Kernel upgrade happens. You reboot to a non-working X first boot with the new Kernel so you might as well just boot into that safe mode and rerun those module assistant commands. Just print out the guide on the NVidia wiki and you'll have all you need. It's not hard, but I just haven't done it myself so can't really give you a how-to from memory. I'm sure you'll find what you need to know. I used to see a Kernel upgrade was there, uninstall NVidia, dpkg-reconfigure back the nv driver, then run the Kernel upgrade and install the NVidia script. That way I never lost X or the ability to boot into the desktop. But with the Debian driver installed that would be cumbersome. You already have the glx driver installed which you'd need to uninstall too to do that. Just boot to a broken X and redo the NVidia module and you'll be fine.
is the best way to learn new things, the only bad thing is when you put it all together and you wonder about the last screw in your hand lol (i remember back in early 90's i saw my dad formatting the hdd of the 486, then few weeks l8er i typed 
