Agenda: The Anti-Catholic Nature of the Society of
"Verbal snapshots"--lots of them. Also official Vatican documents, along with articles from This Rock and Fidelity magazine.
Ecclesia Dei
http://www.vatican.va/holy_father/john_paul_ii/motu_proprio/documents/hf_jp-ii_motu-proprio_02071988
Pope John Paul II declares that Archbishop Lefebvre and other SSPX leaders are automatically excommunicated, because of the consecration of 4 bishops against the pope's express wishes. Furthermore, those who formally adhere to this schism are also excommunicated.
SSPX Cult: Is the Society of St. Pius X a Cult?
Draws on the criteria of several reputable cult experts. Does the SSPX have the characteristics of a manipulative, spiritually unhealthy religious cult? This site says yes.
SSPX: Is the Society of St. Pius X in Schism?
Detailed, sometimes repetitious, examination of canon law. Concludes that Lefebvre was never truly excommunicated but that since his death the SSPX has drifted into schism.