Learning SpamAssassin

[Dan]

Newbie here,

I've filtered email for years with Declude and am adding SpamAssassin  
to my arsenal.  I want to build something from scratch and am having  
problems getting started.  Thing is, all the guides (books, manual,  
web pages) seem to be geared toward using the standard configuration  
or making slight modifications to it - assuming the reader doesn't  
want to see under the hood.  I can't find anything that describes the  
configuration structure that SA uses, its requirements or rules.

I have a sense of 10_, 20_, 50_ file names in terms of controlling  
order and I know how to build SA rules.  But I want to know, if I  
delete every config/rule file, what files would I absolutely have to  
have and what would have to be in them?  For example, do I just need  
a local.cf file with specific global lines and then everything else  
can be body/description/scores, inside any file with any name?  Are  
new rules in new files automatically added when present?

For the purposes of these questions, assume a single configuration  
level will all rules in the same directory.  Any links or  
descriptions would be appreciated.

Thanks,
Dan