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)

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