19908 Michael Barr/Charles Wells: Toposes, triples and theories.
Repr. Theory Appl. Cat. 1 (2005), 1-289.
[Reprint of: Michael Barr/Charles Wells: Toposes, triples and theories. Springer 1983.]

M. Fourman: The logic of topoi. In J. Barwise (ed.): Handbook of mathematical 
logic. North-Holland 1978, 1053-1090.

1154 Robert Goldblatt: Topoi. North-Holland 1986.
[Shall appear 2006 as a Dover edition!]

16545 Luc Illusie: What is a topos? Notices AMS October 2004, 1060-1061.

28697 Peter Johnstone: Topos theory. Dover 2014, 370p. Eur 21.

F. Lawvere: An elementary theory of the category of sets.
Proc. Nat. Ac. Sci. USA 51 (1964), 1506-1510.

17702 F. Lawvere/Robert Rosebrugh: Sets for mathematics. Cambridge UP 2003, 260p. Eur 29.
Set theory based on categories.

24929 Tom Leinster: Higher operads, higher categories. Internet 2003, 410p.

28373 Jacob Lurie: Higher topos theory. Princeton UP 2009, 950p.
Available at the author's website. Very well written.

28698 Saunders Mac Lane: Review of "Topos theory" by P. Johnstone.
Bull. AMS 1/6 (1979), 1005-1014.

16269 Saunders MacLane/Ieke Moerdijk: Sheaves in geometry and logic.
Springer 1992, 630p. Eur 73. A very good book for content and presentation,
but a shame for the publisher: cheaply bound, it looses the first page
20 minutes after arriving. And that at a price of 73 Euro.

Colin McLarty: The uses and abuses of the history of topos theory.
Brit. J. Phil. Sci. 41/3 (1990), 351-375.

Colin McLarty: Elementary categories. Elementary toposes. Oxford UP 1996, 280p. $97.

G. Osius: Categorical set theory: A characterization of the category of sets. 
J. pure & appl. Algebra 4 (1974), 79-119.

29419 Dana Schlomiuk: An elementary theory of the category of topological spaces.
Trans. AMS 149 (1970), 259-278.

Oswald Wyler: Topoi and quasitopoi. 
World Scientific 1991, 280p. 981-02-0153-2. $32.