REFEDS has created a document describing The Value Proposition for Identity Federations.  This is a good place to start if you are considering implementing an identity federation and need more background information and support.

JISC created a Business Case document to help Identity Providers make the case internally for the work required to implement SAML.  The document was written at the beginning of the UK Access Management Federation but is still valid and a valuable resource.
The Jisc Identity Management Toolkit may also be a useful resource.

The GÉANT project created a Federation Policy Template to help new and emerging federations quickly understand the requirements for federation policy and to support an interoperable approach to policy creation and implementation.  Federations are encouraged to use this template.
A downloadable version for editing is also available.

Understanding the Metadata Registration Practices of federations is key to understanding the trust framework for the federation. REFEDS and GÉANT have created a template Metadata Registration Practice Statement and federations are encouraged to use this template.  This statement is a requirement for federations joining eduGAIN:  MRPS-templatev1.pdf (pdf) MRPS-templatev1.docx (word).  Also available in Markdown format on Github.