The leading treatise on Legal Opinions is Fitzgibbon and Glazer on Legal Opinions (Aspen Publishers). Also useful: Legal Opinion Letters: A Comprehensive Guide to Opinion Letter Practice by John Sterba (Aspen Publishers) and Third-Party Legal Opinions (BNA Corporate Practice Series #82).
Most of the significant documents and reports relating to Legal Opinions are collected in the Appendixes at the end of Fitzgibbon and Glazer, such as the Third Party Legal Opinion Report and Legal Opinion Accord of the Section of Business Law of the American Bar Association (also published at 47 Business Lawyer 167 (1991)). Selected TriBar and ABA Section of Business Law reports are posted in the Legal Opinion Resource Center. ABA and Tribar Opinion Committee reports are published in the Business Lawyer and republished in The Collected ABA and TriBar Opinion Reports (available from the ABA Bookstore).
The Legal Opinion Letters Formbook (Aspen) contains sample legal opinions and analysis of ABA, Tribar and state bar reports.
For real property matters, the ABA publishes Real Estate Opinion Letter Practice (ABA Bookstore). For intellectual property, Aspen publishes Intellectual Property Legal Opinions.
Substantive Consolidation: The Committee on Bankruptcy and Corporate Reorganization of the
Association of the Bar of the City of New York published a model "Form of Non-Substantive Consolidation Opinion" as Appendix D to an articled titled, "Structured Financing Techniques," 50 Bus. Law. 527 (1994-1995). The Committee published a shorter model titled, "Form of Substantive Consolidation Opinion," at the end of a "Special Report on the Preparation of Substantive Consolidation Opinions," 64 Bus. Law. 411 (February 2009). This issue is also covered in Legal Opinion Letters: A Comprehensive Guide to Opinion Letter Practice (Aspen) by M. John Sterba, Jr.