Matthias Egger & Douglas Altman 
Systematic Reviews in Health Research [PDF ebook] 

This book has become the standard text on the use of systematic reviews in health research. It is recommended reading for anyone undertaking systematic reviews or needing to understand the principles and methodology used in research papers. It goes beyond explanation, to provide debate on key issues in systematic reviewing.
Written by leading, internationally respected researchers, this is a reference for the informed reviewer but also a valuable resource for those coming to the field for the first time. It is both comprehensive and stimulating — a classic text in its own right.