When I first started studying Thomas in about 2004, I asked about commentaries and was told that the only English language full commentary available was Richard Valantasis’ The Gospel of Thomas (London and New York: Routledge, 1997) and that I was really better off getting a copy of Jacques Ménard’s L’Évangile Selon Thomas (Leiden: E.J. Brill, 1975).
Since then, there have been five commentaries published in English and one in German. They are:
Nordsieck, Reinhard, Das Thomas-Evangelium: Einleitung: Zur Frage des historischen Jesus: Kommentierung aller 114 Logien. (Neukirchen-Vluyn: Neukirchener Verlag, 2004).
DeConick, April D, The Original Gospel of Thomas in Translation, With a Commentary and New English Translation of the Complete Gospel (London: T & T Clark, 2006), together with the companion volume DeConick, April D, Recovering the Original Gospel of Thomas: A History of the Gospel and its Growth (London: T&T Clark, 2005).
Plisch, Uwe-Karsten, The Gospel of Thomas : original text with commentary (Stuttgart: Deutsche Bibelgesellschaft, 2008) – a translation from German.
Pokorný, Petr, Commentary on the Gospel of Thomas: From interpretations to the interpreted. (T&T Clark Jewish and Christians Texts Series. New York: T&T Clark Ltd, 2009).
Hedrick, Charles W., Unlocking the secrets of the Gospel according to Thomas: a radical faith for a new age. (Eugene, Or.: Cascade Books, 2010).
I have linked to my reviews of them, although the material on Plisch is in a post that gives another overview of commentaries, rather than (as yet) having a review of its own. In addition, two books have been published on Thomas this year (Chris Skinner’s What are they Saying About the Gospel of Thomas (Paulist) and Simon Gathercole’s The Composition of the Gospel of Thomas (Cambridge)) and a third, Mark Goodacre’s Thomas and the Gospels: The Case for Thomas’s Familiarity with the Synoptics (Eerdmans) is due out soon. Clearly, there is a significant increase in interest in Thomas!
Update (because I hit publish instead of save draft)
Hedrick’s commentary is suitable for a reader who is not a biblical scholar. The rest assume some knowledge of the discipline. DeConick and Plisch spend a significant amount of time looking at scholarship in the field and provide extensive bibliographies (as does Nordsieck, but in German). Porkorný and Hedrick place less emphasis on this and have much more limited bibliographies. It does not appear that Plisch’s commentary is available in paperback, but all the others are. All are worthy of attention in their own ways, but if I were only to buy two I would choose DeConick and Plisch and would recommend that you also buy DeConick’s companion volume.