Agile: Recommended titles for readers interested in agile development methodologies include Alistair Cockburn's Agile Software Development and Agile Project Management With Scrum by Ken Schwaber. For a more general look at a variety of agile technologies (and techniques), then Integrating Agile Development in the Real World is well worth a look. Many of the techniques adopted by agile teams, such as unit testing, automated builds etc are covered in Steve McConnell's excellent Code Complete. Anyone interested in a very fast intro should also check out our own Agile Development In 30 Seconds.
RUP and UML: An easy introduction to the RUP is provided by Rational Unified Process Made Easy. For readers interested in UML, then either Learning UML 2.0 or Fast Track UML 2.0 are solid introductions.
Object Oriented: There are plenty of books that focus on a particular language implementation of objects, but for a very good language independent guide then Object Thinking by David West is recommended. Of the language-specific titles, Objects First With Java stands out as being different in that it is designed specifically for class-room instruction and uses the BlueJ development environment to teach both Java and object-oriented programming.
Design Patterns: Of course design patterns are a must for anyone learning objects. While the definitive guide remains the Gang of Four book (a review of this classic title is promised some time soon), there are a couple of great introductory books. The first is Design Patterns Explained, which is a good solid introduction. The second is the down-right funky, Head First Design Patterns, which is a fun read but packs plenty of useful content. For a more code-centric and detailed view, then Holub On Patterns is well worth the extra effort.
Other recommended titles: Finally, the books that don't fit neatly into any category. First up is Martin Fowler's classic Refactoring: Improving the Design of Existing Code, a must read for every active developer. Facts and Fallacies of Software Engineering, by Robert Glass, is packed full of wisdom and good sense. Again, this is a book that every developer, (and project manager), should read. Finally, did we mention Code Complete?