ICD10 - WHO International Classification of Diseases 

OPCS 4.4 - procedure coding 
(The Clinical Audit department has both books and a spreadsheet for ICD10 and a spreadsheet for OPCS.) 

BNF is available online via the GHNHSFT library website

Bandolier - a useful collection of articles about evidence based healthcare and explanations of the interpretation of medical statistics

Evidence Based Medicine - a useful collection of reference sources and explanations (requires Athens signon) 
 
Office for National Statistics - UK statistics including health data 
 
Gobbledygook - Plain English Campaign 1984 - ISBN 0-04-827107-1 
 
The Visual Display of Quantitative Information (2nd Edition) - Tufte, Edward R. (2001) - Cheshire, CT: Graphics Press - ISBN 0961392142 - A classic on the subject of clear, clutter-free presentation and the omission of graffiti.

Misleading graph - Wikipedia article

How to Lie With Statistics - Darrell Huff (1954) - best selling book on statistics
 
Layout guidelines for documents and presentations

Wikipedia's article on Clinical Audit