o'reilly & associates
They probably publish the most carefully and most professionally written computer books. You will find something on almost every topic and several books have become the number one reference in their field.
www.ora.com

dpunkt
The German analogon to O'Reilly when it comes to professional quality. Very interesting books on programming languages and also on advanced topics like firewalls, etc.
www.dpunkt.de

galileo press
They publish carefully written computer books on very interesting topics like Java, C++, XML, DHTML, Flash, and Lego Mindstorm.
www.galileo-press.de

econ
The yellow paperbacks of the german publishers are written very carefully and full of helpful screen-shots. They are my preferred recommendation for books about MS Office applications.
www.rabbitsoft.de

amazon
My favourite online book store. Especially for computer literature. Free delivery to Switzerland when ordering at the german branch.
www.amazon.com    www.amazon.de

c't
In my opinion, the one and only german computer magazine. The corresponding website includes uptodate news, interesting links, a lot of shareware and much else.
www.heise.de/ct