For a introduction:

1) Notes on Logic and Set theory (cap. 1, cap 3)
P.T. Johnstone. 

2) Locally Presentable And Accessible Categories by J Adamek J Rosicky (Cap. 3 & cap. 5)

FOr a comprehensive view:

1) Sketches of an Elephant: A Topos Theory Compendium (VOl 2, cap D1)

3) B. Jacobs, Categorical Logic and Type Theory, Studies in Logic and the Foundations of Mathematics 141