The very useful Peregrine e-discussion group (previously the Pelican Initiative) run by the Better Evaluation group, is a source of rich discussion. Its members share useful documents relevant for anyone interested in evaluations. 

The most recent document: Writing Terms of Reference for an Evaluation: A How To Guide, is a useful contribution. 

Published by the Independent Evaluation Group of the World Bank (IEG) the guide helps to bridge the gap between commissioners and evaluators. It spells out how to write clear, concise, and well-structured ToRs that help to communicate both the overall purpose of the evaluation, as well as to set out practical guidelines on the resources available and methods that would be expected from anyone bidding to carry out the work. 

A very useful read for anyone involved in the process of bidding for, or conducting evaluations, as well as those commissioning them.