Ilias Lazaridis wrote:

> Where can I find the following information?:
> 
>   * An UML diagramm (or another visual representation) of the ruby 
> class-model.

I've generated this one automatically:

http://flgr.0x42.net/class-graph.png

Note that it is quite large and not too pretty, though.

>   * A deep (but compact) description of the reflection/introspection api's.

Well, the Pickaxe book (slightly outdated version is available online 
for free, new issue with lots of new material can be ordered online) 
contains that among much other information and is a pretty interesting 
read. See

http://www.rubycentral.com/book/ (first issue)
http://phrogz.net/ProgrammingRuby/ (first issue with Wiki for changes)
http://www.pragmaticprogrammer.com/titles/ruby/index.html (second issue)
http://www.amazon.com/exec/obidos/tg/detail/-/0974514055/ (second issue)

I'd recommend ordering the second issue from the pragmatic guys as it 
contains detailed information that will be useful for evaluating Ruby in 
depth.