The book that I most often wish existed, because co-workers often ask me for it, is a good introduction to object-oriented design.

If you don't care what programming language it uses, have a serious look at the Inform Designer's Manual, by Graham Nelson. Skip the first chapters about the language syntax and go straight to the section on the object model. (Yes, you can easily follow the section on the object model without having read the earlier chapters on syntax; trust me on this.) This is without qualification the *best* computer-technical book I have ever seen, and it is an *excellent* introduction to object-oriented programming.

It's not, of course, the only OOP book you will ever need. After you finish it you will need books that get down to nuts and bolts in the language you're going to be using (assuming that isn't Inform). This is presumably where the books with details of how Perl OO works would come in. Nevertheless, as an introduction to OOP in general, the Designer's Manual is fantastic.

Incidentally, the text of the book is freely available on the author's website, so if you want to preview it before purchasing a copy of the print edition, you can. The section on the object model starts here, although the real meat is in chapter 2, which really can just about stand on its own.