Provability and reflection

[PB]

After trying hard at finding something foundational 
on the theory of reflection, at last I found this:

http://wwwmath.uni-muenster.de/math/inst/logik/publ/pre/2.html

These are the lecture notes for a course called "Provability 
and reflection" kept for ESSLI97.

Abstract on:
http://www.lpl.univ-aix.fr/~esslli97/abstracts.frame-en.html#logic5

Any opinion? Any other pointer to some paper which may
help to get in touch with the theoretical and foundational 
aspects of reflection?

Pietro