THE BIBLICAL REVIEW BLOG: Pseudepigrapha Saturday: Eldad and Modad. Actually this one came out last Sunday.
It is correct that Eldad and Modad is only quoted explicitly by title once, in the Shepherd of Hermas. But it may be mentioned and quoted in other places and may even be quoted in the New Testament in the Epistle of James. It was treated briefly by E. G. Marshall in Charlesworth's Old Testament Pseudepigrapha, 2:463-65 and then again more recently and at greater length by Richard Bauckham in Old Testament Pseudepigrapha: More Noncanonical Scriptures (Ed. Bauckham, Davila, Panayotov; Eerdmans, 2013), 1:244-56 (and see the bibliography there).
This Pseudepigrapha Saturday blog post appears just to be introductory, so perhaps William Brown will be following it up with more details in future posts.
Cross-file under Old Testament Pseudepigrapha Watch.