perltoot (Perl documentation). Tom Christiansen.
"Tom's object-oriented tutorial" gives an excellent treatment of object orientation and, specifically, OO with Perl. A must-read.
From ftp://rtfm.mit.edu/pub/usenet/comp.object. One of the best FAQ compilations around.
Object-Oriented Modeling and Design. J. Rumbaugh, M. Blaha, W. Premerlani, F. Eddy, and W. Lorensen. Prentice-Hall, 1991.
An excellent treatment of object orientation, especially as translated to a programming language. Also contains good comparisons of OO with other software methodologies.
Design Patterns: Elements of Reusable Object-Oriented Software. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. Addison-Wesley, 1994.
This book is a catalog of commonly used patterns of interacting objects (independent of language). Even if the patterns themselves are sometimes intuitive, the very act of giving them a name enriches an object practitioner's vocabulary.
Bringing Design to Software. Terry Winograd. Addison-Wesley, 1996.
Among other things, this book examines several highly successful software products and argues that user-oriented design is the best software methodology. (None of the products that have really sold have worried particularly about object-oriented programming.) Interesting and persuasive.
The Mythical Man-Month. Frederick P. Brooks. Addison-Wesley, 1995.
Object-Oriented Software Engineering: A Use Case Driven Approach. Ivar Jacobson. Addison-Wesley, 1994.