>> Ruby seams like a nice place to start (I now mainly use perl and
>> -some- tcl/tk), but I am -first- looking for a good book on 
>> OO-programming itself. (Not really the language, but the basic
>> 'way of thinking' of OO).
>> 
>> Anybody any ideas?

There's one book I have at work that I might 
recommend. I'll go take another look at it first. 
Also, Eckel's "Thinking in..." books are quite 
good, but they are targeted at specific 
languages.

One trick that really helped me "get" objects was 
to think about how to write a sports or role-
playing game. Perhaps a baseball game, or 
Dungeons and Dragons, or choose your own 
favorite.

Now consider that each being in the game is an 
object. Each significant item (like a sword) is 
an object. Think about how each object has 
properties (e.g. accuracy), and performs actions 
(e.g. shoot). 

Think about how different objects share the same 
class (e.g. 'Player'), but may have different 
specific attributes (Michael Jordan has a higher 
'accuracy' value than someone else). 

Think about how certain classes might share 
common base classes, like in football you might 
have 'lineman' and 'receiver' classes that both 
inherit from 'offensiveplayer'.

Hope it helps...

Kevin