So you can see what a security do in a bookstore at night... read books :) This is Luxor at Wenceslas square. One of my favorite bookstores in Prague. They have a wide selection of photographic books (and other artistic books).
I wanted to show you this post about new technical library at Dejvice. That library is awesome and if you speak Czech you'll also enjoy the post very much. (Or... you can use Google to translate it, here's English version)
PS: I must say the whole library system in Prague is superb. I'm using it with a great pleasure for 20 years now.