"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.
An excellent treatment of object orientation, especially as translated to a programming language. Also contains good comparisons of OO with other software methodologies.
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.
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.