This is a very vague question, but a question nonetheless. I am becoming increasingly more interested in what can be vaguely categorized as Mathematical Philosophy, or more specifically perhaps, Metamathematics, that is, the study of Mathematics itself! I.e. Theorems about theorems.
Examples of such theorems would be the Incompleteness theorems, Hilbert's tenth problem, the Continuum Hypothesis etc... I don't know what 'branch' of Mathematics to bracket these theorems into but I am sure that those reading this post know exactly what I mean.
My question is, can anybody recommend any books that would introduce me to this 'branch' of Mathematics? I don't mean Mathematical Philosophy for laymen, but a proper introduction to the theory.
Many thanks, Elie.

Peter Smith, who uses this forum and wrote a very accessible but rigorous book on Gödel's incompleteness theorems, has a great annotated bibliography here:
http://www.logicmatters.net/tyl/
If you already know elementary symbolic logic, you can look at the higher-level references. (And Andres, who commented on your post, is a working mathematical logician. We are blessed to have these people active in this community.)