Dodd’s The apostolic preaching and its developments covers what he thinks the kerygma is in detail, especially early in the book. Not so much the act of preaching but the message. Not so much teaching or exhortation but gospel proclamation.
Thiselton’s Hermeneutics Notes that Bultman and Fuchs both make a big deal of this. See subject index therein for kerygma. Bultman is contrasting with “banal teaching.”
Baird, William. “What Is the Kerygma? A Study of I Cor 15:3-8 and Gal 1:11-17.” Journal of Biblical Literature 76, no. 3 (1957): 181–91. https://doi.org/10.2307/3261567.
McDonald, James I. H. Kerygma and Didache: The Articulation and Structure of the Earliest Christian Message. Society for New Testament Studies Monograph Series. Cambridge: Cambridge University Press, 1980. https://doi.org/10.1017/CBO9780511520419.
Thiselton, Anthony C. Hermeneutics: An Introduction. Grand Rapids, Mich: W.B. Eerdmans Pub. Co, 2009.