A Still, Small Voice: A Practical Guide on Reported Revelations by Fr. Benedict Groeschel, C.F.R. - $11.95. This is a useful volume on discernment, which has much to commend it. However, the author does appear to give Medjugorje more credence than it actually deserves and so some caution in recommending it is necessary.
A Still, Small Voice : A Practical Guide on Reported Revelations - £7.53 - UK & Europe
Authenticity: A Biblical Theology of Discernment by Fr. Thomas Dubay, S.M. - $14.95. Although not about apparition discernment as such, this is nevertheless a good overview of discernment, as indicated in the Scriptures. Brings home the point that any "revelation" must be judged by the Church, and that this has been the consistent approach since New Testament times.
Authenticity : A Biblical Theology of Discernment - £9.42 - UK & Europe
Review of AUTHENTICITY. A Biblical Theology of Discernment. Revised Edition. By Thomas Dubay, S.M. (Ignatius Press, 1996), 260 pp. PB, by Kenneth Baker, S.J. - scroll down page at:
Apparitions: Mystic Phenomena and What they mean by Kevin Orlin Johnson, Ph.D. - $25.00, (392 pages). A very good guide to all things mystical, and not just Marian apparitions. Has sections on discernment, and is thus a good reference book for anyone wanting to know what the Church really thinks about these matters. Recommended for those who want an in-depth study of mystical phenomena.
Apparitions: Mystic Phenomena and What they mean - £17.51 - UK & Europe