These tutorials were authored by Waldo and Sam from Counter-Map, which is either down atm or gone.
The first tutorial/example map show you how to create one-way glass--using the NULL brush and a little ingenuity, and the second tutorial shows you how to make it breakable.
I'm posting them here because I couldn't find them anywhere else.
It's a very simple effect but it's nice. Just for the realisim, make the lights dimmer in the... dang it... forgot the name... observation room? Well you get my meaning. But since this is an example map there is no need for that.
Because there was a guy asking 'bout 1-way glass in a recent thread, I decided to post it out of my archive of tutorials/example maps.