Chris Rayns, Roland Grant et.al., Multilevel Security and DB2 Row-Level Security Revealed. IBM Redbook SG24-6480-00. Good introduction to MLS on Z/OS. Explains categories and levels well. The networking part is interesting as it poses a fundamental trust problem for label based security. see pages 127 and 282ff. for an explanation of mandatory access control.
Christel Eckert, IT-Sicherheit. Good but short explanations of label based security, Bell-LaPadula models etc. Shows that label based security also needs the concept of trusted processes.
Andrew Tanenbaum, Jorrit Herder, Herbert Bos, May 2006, Can we make Operting Systems Reliable and Secure? with armored system examples and a discussion of singularity
> Singularity OS . A research OS with high isolation concepts