Several people have mentioned that these books are great study material for the CISSP certification:
http://www.wittys.com/textfiles.html#Rainbow
Update 04/30/2008:
I found a more complete set of DoD Rainbow books on WindowSecurity.
Here are the books available:
NCSC-TG-004: Teal Green book
Glossary of Computer Security Terms
NCSC-TG-005: Red book
Trusted Network Interpretation
NCSC-TG-006: Orange book
A Guide to Understanding Configuration management in Trusted Systems
NCSC-TG-007: Burgundy book
A Guide to Understanding Design Documentation in Trusted Systems
NCSC-TG-008: Dark Lavender book
A Guide to Understanding Trusted Distribution in Trusted Systems
NCSC-TG-009: Venice Blue book
Computer Security Subsystem Interpretation of the Trusted Computer System Evaluation Criteria
NCSC-TG-010: Aqua book
A Guide to Understanding Security Modeling in Trusted Systems
NCSC-TG-011: Dark Red book
Trusted Network Interpretation Environments Guideline - Guidance for Applying the Trusted Network Interpretation
NCSC-TG-013: Pink book
Rating Maintenance Phase Program Document
NCSC-TG-014: Purple book
Guidelines for Formal Verification Systems
NCSC-TG-015: Brown book
A Guide to Understanding Trusted Facility Management
NCSC-TG-016: Yellow Green book
Writing Trusted Facility Manuals
NCSC-TG-017: Light Blue book
A Guide to Understanding Identification and Authentication in Trusted Systems
NCSC-TG-018: Light Blue book
A Guide to Understanding Object Reuse in Trusted Systems
NCSC-TG-019: Blue book
Trusted Product Evaluation Questionnaire
NCSC-TG-020A: Grey book
Trusted UNIX Working Group (TRUSIX) Rationale for Selecting Access Control List Features for the UNIX System
NCSC-TG-021: Lavender book
Trusted Database Management System Interpretation of the TCSEC (TDI)
NCSC-TG-022: Yellow book
A Guide to Understanding Trusted Recovery in Trusted Syst
NCSC-TG-023: Bright Orange book
A Guide to Understanding Security Testing and Test Documentation in Trusted Systems
NCSC-TG-025: Green book
A Guide to Understanding Data Remanence in Automated Information Systems
NCSC-TG-026: Hot Peach book
A Guide to Writing the Security Features User’s Guide for Trusted Systems
NCSC-TG-027: Turquiose book
A Guide to Understanding Information System Security Officer Responsibilities for Automated Information Systems
NCSC-TG-028: Violet book
Assessing Controlled Access Protection
NCSC-TG-029: Blue book
Introduction to Certification and Accreditation Concepts
NCSC-TG-030: Light Pink book
A Guide to Understanding Covert Channel Analysis of Trusted Systems
CSC-STD-001-83: Orange book
DoD Trusted Computer System Evaluation Criteria
CSC-STD-002-85: Green book
DoD Password Management Guideline
CSC-STD-003-85: Light Yellow book
Computer Security Requirements - Guidance for Applying the DoD TCSEC in Specific Environments
CSC-STD-004-85: Yellow book
Technical Rationale Behind CSC-STD-003-85: Computer Security Requirements - Guidance for Applying the DoD TCSEC in Specific Environments
NCSC-TG-001: Tan book
A Guide to Understanding Audit in Trusted Systems
NCSC-TG-002: Bright Blue book
Trusted Product Evaluation - A Guide for Vendors
NCSC-TG-003: Neon Orange book
A Guide to Understanding Discretionary Access Control in Trusted Systems