22152 Jiri Adamek/Horst Herrlich/George Strecker: Abstract and
concrete categories. Dover 2009, 520p. Eur 19.

21916 Steve Awodey: Category theory. Oxford UP 2010, 310p. Eur 37.

27064 Steve Awodey/Thierry Coquand/Vladimir Voevodsky (ed.): Homotopy type theory.
IAS Princeton 2013, 49op. Available on https://homotopytypetheory.org/book.

19908 Michael Barr/Charles Wells: Toposes, triples and theories.
Repr. Theory Appl. Cat. 1 (2005), 1-289.

19909 George Bergman: Categories and functors. Internet 1998, 56p.
Chapter 6 of the authors "Invitation to general algebra and universal
constructions.".

F. Borceux: Handbook of categorical algebra. 2 volumes.
Cambridge UP 1994, 350+440p. 0-521-44178-1, -44179-X. Pds. 45+50.

F. Borceux/G. Janelidze: Galois theories. Cambridge UP 2001, 340p. Pds. 50.
A categorical approach to Galois theories up to Grothendieck's Galois theory
and topoi.

13024 Anna Bucalo/Giuseppe Rosolini: Lifting. Internet ca. 1998, 11p.

W. Burgess: The meaning of mono and epi in some familiar categories.
Can. Math. Bull. 8 (1965), 759-769.

15379 Hartmut Ehrig a.o.: Mathematisch-strukturelle Grundlagen der
Informatik. Springer 2001, 620p. Eur 33.

Samuel Eilenberg/Saunders Mac Lane: General theory of natural equivalences.
Trans. AMS 58 (1945), 231-294.

1133 Josef Eschgfäller: Der Begriff der Stufung. Manuskript, ca. 1978.

M. Grandis: Category theory and applications. World Sci. 2018, 300p.

5944 Martin Jurchescu: Sulla teoria delle categorie.
Quaderno CNR 1974.

22258 Masaki Kashiwara/Pierre Schapira: Categories and sheaves.
Springer 2010, 500p. Eur 94.

Ralf Krömer: Tool and object - a history and philosophy of category theory.
Birkhäuser 2007, 370p. Eur 106.

19910 Günter Landsmann: Introduction to category theory. Internet 2007, 32p.

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

29286 Tom Leinster: Basic category theory. Arxiv 2016, 180p.

4425 Saunders MacLane: Kategorien. Springer 1972.
[English: Categories for the working mathematician. Springer 1998. DM 119.]

19872 Alfio Martini: Elements of basic category theory.
Internet TU Berlin 1996, 160p.

21831 Claudia Menini: Categories. Internet 2009, 80p.

B. Mitchell: Theory of categories. Academic Press 1965.

20088 Hans Opolka: Besprechung des Buches "Tool and object" von Ralf Krömer.
Math. Sember. 55 (2008), 92-97.

5943 Bodo Pareigis: Kategorien und Funktoren. Teubner 1969.

5746 Dieter Pumplün: Kategorien. 5738 Laugwitz, 177-192.

16260 Dieter Pumplün: Elemente der Kategorientheorie. Spektrum 1999, 310p.

27308 Emily Riehl: Category theory in context.
Dover 2016, 260p. Eur 22. PDF free from the author.
See also the author's very beautiful website.

23490 William Schmitt: A concrete introduction to categories. Internet. 60p.

5945 Horst Schubert: Kategorien. 2 Volumes. Springer 1970.

1141 Zbigniew Semadeni/Antoni Wiweger: Einfuehrung in die Theorie der 
Kategorien und Funktoren. Teubner, Leipzig 1979.

27321 Michael Shulman: Set theory for category theory. Arxiv: 0810.1279 (2008), 39p.

28665 David Spivak: Categories as mathematical models.
Arxiv 1409.6067 (2014), 21p.

Paul Taylor: Practical foundations of mathematics.
Cambridge UP 1999, 570p. 0-521-63107-6. $80.

19949 R. Walters: Categories and computer science.
Cambridge UP 2006, 170p. Eur 25.