TITLE: Reference on internal categories and externalization
QUESTION [7 upvotes]: I'm looking for a reference on internal categories and externalization of internally defined notions.
The nlab has a stub on externalization (more details are available under small fibration) and the page on internal categories gives enough of an introduction that I can sketch most internal notions, but I could really use a concise introduction to internal categories and externalization, and if possible the relationship between internalization and externalization. Are they adjoint in some sense?
I'm fine assuming a background of $2$-category theory, so talking about the $2$-category of internal categories in a category with pullbacks etc. would make sense, but ideally the reference would assume no familiarity with internal category theory or externalization. Any assistance is appreciated.

REPLY [7 votes]: For what it's worth, I should add my comment as an answer. Chapter 1 of Bart Jacobs' book Categorical logic and type theory (Studies in Logic and the Foundations of Mathematics 141 (1999), (author's page, publisher page, pdf)) is a good intro to fibred category theory, and chapter 7 has an intro to internal category theory, and it links the two. Chapter 9 does some more advanced fibred category theory.