This briefing paper by Brown (2001) is a useful straightforward guide to some of the key issues around assessment.

This paper by Gibbs & Simpson (2002)  How assessment influences student learning – a conceptual overview

is helpful for mapping out research into this area.

A key aspect of assessment is the feedback that students receive before, during and after they engage in a particular assessment task.  The Feedback section of the Effective Teaching website addresses this issue in more detail.

This booklet "Assessment Matters" (available in both WORD and PDF format) was produced by CELT and contains a number of vignettes of assessment practices at Lancaster University. 

The Higher Education Academy has recently released a short video which looks at staff & student perceptions of marking criteria and assessment methods . 

This link takes you to more selected references on assessment.