question on writing extensible code

[eric-yrHdaQSNc4gdnm+yROfE0A at public.gmane.org (Eric Chadbourne)]

any suggested reading on this?  i'm particularly interested allowing 
customers to write their own modules and in handling upgrades.  i've 
gotten some ideas from several free software projects but it would be 
enjoyable to read up on the topic.

thanks for any tips,
eric c.