I've lost track of the number of Oracle books I've bought over the years, but those below stand out as the being the best I've seen (by a considerable margin over many of the others). I recommend each for your bookshelf.

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cover An excellent introduction to the wonderful world of the cost based optimizer
cover The definitive book for Oracle developers to build better applications and avoid the common pitfalls that haunt many Oracle applications in the marketplace.
cover Tom Kyte's most recent book. An excellent discussion of the important decisions to make when building an application on the Oracle database.
cover Want to know about the important operational timing data and response analysis from one of the best in the business. Then look no further than this book by Cary Milsap.
cover Easily the best 8i book around. A complete and comprehensive description of all the 8i features from a DBA perspective.
cover Because its a Wrox book you get the customary miniscule font, and the author's eyes on the cover following you around the room, but suddenly building a successful Oracle project got a whole lot easier. You give the Johnathan Lewis book to your DBA, and you give this one to your developers (and your DBA). When they can all succesfully tell you how many commas appear on page 347 :-) then your project is ready to go. Don't let a developer loose on your project until they've read this book.
cover It will take a few reads of this books before you understand it all, such is the level of depth to which it delves into the bowels of Oracle. But at the end, you'll have a far better appreciation of what a complex beast the Oracle kernel is. Unfortunately, because Oracle have been changing internals with such ferocity, some of the details are now out of date, but its still the best book of its type.
cover An excellent description of the Unix structures that Oracle utilises and how to best leverage both Unix and Oracle features to maximise performance