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
