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PostgreSQL 
Practical PostgreSQL (J. Worsley, J. Drake) [Mirror] [Mirror] [Mirror] [Mirror] [Mirror]
PostgreSQL: Introduction and Concepts (B. Momjian) [Mirror] [Mirror] [PDF]
Writing PostgreSQL Applications (B. Momjian) PostgreSQL Hardware Performance Tuning (B. Momjian)
PostgreSQL SQL Syntax and Use Adventures in PostgreSQL [Episode 2] [Episode 3]
PostgreSQL Technical Documentation