Rachel Cordone's Unreal Development Kit - Game Programming with UnrealScript (Packt Publishing, 2011) is right now the best reference on the subject. The rapid release schedule of UDK makes it difficult to write about and teach, and Cordone's book uses the December 2011 version of UDK, which was released the same month. As far as I know, that makes it the only up to date reference book. More than that, it provides value to experienced coders by adding UnrealScript to their arsenal, then goes even further by adding UDK's Kismet and multiplayer development to the experienced UnrealScript writer's skills.
With more than 420 pages of instructional content, it looks a bit daunting at first, but a fast skim through the text shows Cordone is having fun with her material. Her gamer's personality comes through and the text is conversationally informal and refreshing for the reader. Overall she maintains a comfortable environment where the beginner experiences more enjoyable progress and less anxiety over the material.
The book is best for beginner game developers. or beginners to UnrealScript, but if readers have had even a basic programming course, or ever worked with any kind of scripting at all, they'll see rapid progress in the Unreal environment.
For a point of reference, I came to video game development from a multimedia and web background, and I have a little experience with other scripting, and a lot of experience in the UDK editor and with Kismet. As a game artist and level designer, I sense that some of the stuff in here I might never use, and it would be an advantage if a few more of the complex functions had descriptions of why they're used, or in what situation, from a game play standpoint instead of a coder's motives.
However, it's not difficult to advance. At a regular but relaxed pace, I was able to complete the first two chapters in about a weekend, and I'll probably be done with Chapter 3 (on the engine class hierarchy) by next weekend. I'm drooling at the Kismet chapters coming up. There's some great code on handling meshes and materials that I can see interesting uses for. And hey, who among us hasn't dreamed of making their own weapons and their own enemies? There's also a chapter on the fundamentals of setting up and working with the client/server relationship, for those who want to team up with friends to beat down their own creations in a multiplayer game.
It's been six or more years since there was a comprehensive book written on this subject, and the development and mod communities have been asking for an UnrealScript reference for the entire release life of UDK. In Cordone's book they have it, using the most updated version of the engine. This book is at it's most useful and valuable right now. So don't wait around. Go get it!
As of this writing, the e-book is on sale right now 15% off.
Full/fair disclosure. The reviewer was granted a free copy of the e-book, but he doesn't shill for something he wouldn't use or didn't believe in.
Unreal Development Kit Game Programming with UnrealScript: Beginner's Guide