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An Essential Java Library
By an Amazon.com customer
Effective Java(TM) Programming Language Guide (The Java Series)Effective Java(TM) Programming Language Guide (The Java Series) by Joshua Bloch
Buy new: $40.40 / Used from: $14.95
The bible of best practices: Every programmer should own this
Expert One-on-One J2EE Development without EJBExpert One-on-One J2EE Development without EJB by Rod Johnson
Buy new: $26.39 / Used from: $7.94
A provocative study of enterprise-scale Java by the creator of Spring; makes J2EE not such a dirty word
Concurrent Programming in Java(TM): Design Principles and Pattern (2nd Edition) (The Java Series)Concurrent Programming in Java(TM): Design Principles and Pattern (2nd Edition) (The Java Series) by Doug Lea
Buy new: $52.50 / Used from: $25.00
Transcending Java in scope, this crunchy tome establishes patterns applicable to any domain where concurrency is important -- not just multi-threaded programming, but also services, databases, etc.
Processing XML with Java(TM): A Guide to SAX, DOM, JDOM, JAXP, and TrAXProcessing XML with Java(TM): A Guide to SAX, DOM, JDOM, JAXP, and TrAX by Elliotte Rusty Harold
Buy new: $37.79 / Used from: $19.36
XML is increasingly important, and Java is the most capable language for working in it: Here's your roadmap
Hibernate in Action (In Action series)Hibernate in Action (In Action series) by Christian Bauer
Buy new: $29.67 / Used from: $19.65
Hibernate has emerged as one of the most complete, congenial and configurable ORM technologies to date (only covers version 2, though -- you may want to wait for the next edition)
Java in a Nutshell, Fourth EditionJava in a Nutshell, Fourth Edition by David Flanagan
Buy new: $26.37 / Used from: $0.79
Though less frequently consulted since I started using Eclipse, I find this reference nevertheless indispensable
Pro JSP, Third EditionPro JSP, Third Edition by Simon Brown
Buy new: $59.99 / Used from: $3.99
A concise reference covering the servlet specification, JSP, and JSTL
Java RulesJava Rules by Douglas Dunn
Buy used from: $2.29
An exegesis of the JVM and language specifications: Not light reading, but edifying