A very incomplete list of papers and books on groupoids (or that mention groupoids) arranged chronologically (by publication date)

1961

A.H. Clifford and G.B. Preston, The Algebraic Theory of Semigroups, Volume I, American Mathematical Society, Providence, R.I., 1961.

    In this book on semigroups, groupoids are discussed briefly.  The authors use the term "groupoid" to be a nonempty set with a binary operation and the term "partial groupoid" to be a nonempty set with a binary operation defined on a nonempty subset of S x S.  The term "Brandt groupoid" is used to discuss the algebraic system introduced by Brandt.

Grothendieck, A., Techniques de construction et theoremes d'existence en geometrie algebrique III: preschemas quotients, Seminaire Bourbaki 13e annee 1960/61 no. 212 (1961).

    In Grothendieck's work on moduli spaces, groupoids occur as generalizations of equivalence relations.

1962

Hoehnke, H.J., Zur Theorie der Gruppoide: I, Math. Nachr. 24 (1962), 137-168.

Hoehnke, H.J., Zur Theorie der Gruppoide: II, Math. Nachr. 24 (1962), 169-179.

Hoehnke, H.J., Zur Theorie der Gruppoide: III, Acta Mat., 13 (1962), 91-100.

Hoehnke, H.J., Zur Theorie der Gruppoide: IV, Monats. der Deut. Akad. Wiss., 4 (1962), 337-342.

Hoehnke, H.J., Zur Theorie der Gruppoide: V, Monats. der Deut. Akad. Wiss., 4 (1962), 539-544.

Keefe, Audrey Ann, Coset expansions and homomorphisms of groupoids and quasi-groups, M.A. Thesis, University of Colorado, 1962.

1963

Ehresmann, C., Groupoides sous-inductifs, Annales de L'Institut Fourier, Grenoble 13 (1963), 1-60.

Hasse-Michler, Uber die Einbettbarkeit von Kategorien in Gruppoide, Math. Nachr., 25 (1963), 169-177.

Hoehnke, H.J., Zur Theorie der Gruppoide: VI, Math. Nachr., 2 (1963), 191-198.

Hoehnke, H.J., Zur Theorie der Gruppoide: VIII, Publ. Fac. Sci. Univ. Purkyne, Brno., 443 (1963), 195-222.

Hoehnke, H.J., Zur Theorie der Gruppoide: IX, Monats. Deut. Akad. Wiss., 5 (1963), 405-411.

Rinow, W., Uber die Vervollstandigung induktiver Gruppoide, Mathematische Nachrichten 25 (1963) 199-222.

    This paper is apparently the first paper in the "German school of ordered groupoid theory."

1964

Higgins, P.J., Presentations of groupoids, with applications to groups, Proceedings of the Cambridge Philosophical Society, 60 (1964) 7-20.

Hoehnke, H.J., Zur Theorie der Gruppoide: VII, Math. Nachr., 27 (1964), 289-298.

1965

A.H. Clifford, Partial Homomorphic Images of Brandt Groupoids, Proceedings of the American Mathematical Society, 16 (1965), 538-544.

C. Ehresmann, Categories et Structures, Dunod, Paris, 1965.

    Groupoids are mentioned in passing as a particular type of category where every morphism is invertible.

Schein, B.M., On the theory of generalized groups and generalized heaps in The theory of semigroups and its applications I,  University of Saratov, Saratov, 1965, 286-324 (Russian).

    Written in Russian, this paper apparently did not come to the attention of most semigroup theorists until its translation appeared in 1979.

Uesugi, T., A note on the topological groupoid associated with a pseudogroup of transformations, Bull. of Kyushu Inst. of Technology, 12 (1965), 35-40.

Uesugi, T., On the relations between the groupoid and the pseudogroup of transformations, Bull. of Kyushu Inst. of Technology 12 (1965) 41-55.

1966

Hasse, M., and Michler, L., Theorie der Kategorien, VEB Deutscher Verlag der Wissen Schaften, Berlin, 1966.

    This book contains a chapter on ordered groupoids and categories.

Mackey, G.W., Ergodic theory and virtual groups, Math. Ann. 166 (1966) 187-207.

Pradines, J., Theories de Lie pour les groupoids differentiables: relations entre propretes locales et globales, C. R. Acad. Sce. Paris Ser. A 263 (1966) 907-910.

1967

Brown, Ronald, Groupoids and Van Kampen's theorem, Proceedings of the London Mathematical Society (3) 17 (1967) 385-340.

    States and proves the van Kampen theorem for the fundamental groupoid of the union of two spaces.

Gabriel, P., and Zisman, M., Calculus of fractions and homotopy theory, Springer Verlag, Berlin, 1967.

    An introduction to algebraic topology, groupoids are found as an application of the theory of categories of fractions, in the study of the Poincare groupoid of the simplicial set X, and are used to give proofs of various exact sequences occurring in algebraic topology.

Pradines, J. Theorie de Lie pour les groupoids differentiables.  Calcul differentiel dans la categorie des groupoides infinitesimaux. C.R. Acad. Sci Paris Ser. 264 (1967), 245-248.

V.V. Vagner, Diagrammable Semigroupoids and Generalized Groupoids, Izv. Vuzov. Mat. 10 (1967), 26-39.

    In Russian.

1968

Brown, Ronald, Elements of Modern Topology, McGraw Hill, 1968.

    In the preface, Brown states that in 1965 he discovered the utility of groupoids for computing the fundamental group.  Because of this, he decided to devote four chapters to the development of groupoids for use in computing fundamental groups.  He states that "The most important feature here is probably the way in which the computations of the fundamental group derive from a general property of the fundamental groupoid ..."  It is also interesting to note that writing this book started Brown doing research on groupoids.

Pradines, J., Geometrie differentielle au-dessus d'un groupoide, C.R. Acad. Sci Paris Ser. 266 (1968), 1194-1196.

Pradines, J. Troisieme theoreme de Lie pour les groupoides differentiables, C.R. Acad. Sci. Paris Ser. 267 (1968).

Segal, G., Classifying spaces and spectral sequences, Publ. Math. I.H.E.S. 34 (1968) 105-112.

    "Popularized" the notion of a nerve of a groupoid.

J.-C. Spehner, Sur les groupoides partiels librement engendres par un groupoide partiel. C.R. Acad. Sci. (Paris) A, 267 (1968), 531-533.

Westman, J., Harmonic analysis on groupoids, Pacific J. Math 27 (1968) 621-632.

1969

Bruter, C.P., An Algebraic Structure for Graphs on the Factorization of Groupoids, American Mathematical Monthly, 76 (1969), 658-661.

V.T. Kulik, Simple f-semi-groupoids, Issled. Alg. Izdat. Saratov. Univ. Saratov, (1969) 23-31.

    In Russian.

Ordman, E.T., Amalgamated free products of groupoids, Ph.D. Dissertation, Princeton University, 1969.

J.-C. Spehner, Groupoides partiels libres et produits libres de groupoides partiels. C.R. Acad. Sci. (Paris) A., 268 (1969(, 1005-1007.

Westman, J., Cohomology for ergodic groupoids, Trans. Amer. Math. Soc., 146 (1969) 465-471.

1970

Brown, Ronald, Fibrations of groupoids, J. Alg. 15 (1970) 103-132.

Michler, L., Bemerkungen uber induktive Gruppoide, Mathematische Nachrichten 43 (1970), 47-64.