IDL

These are all the third-party books about IDL. There are plenty of PDF “books” distributed with IDL (some of them are also available in hardcopy from RSI).

IDL Programming Techniques by David Fanning
Introductory to intermediate level general-purpose book about IDL. If you are going to get one book to get started in IDL, I would recommend this book.
Practical IDL Programming by Liam E. Gumley
Introductory to intermediate level general-purpose book about IDL.
An Introduction to Programming with IDL by Ken Bowman
This book is geared towards a new user of IDL without programming experience. It covers the necessary topics to get started in IDL basic variable concepts, analysis, file input/output, and direct graphics visualizations including many exercises for these topics. I think it achieves its goal very well, but don’t look here if you want to take your programming beyond basic analysis and visualization. See my review for more information.
Application Development with IDL: Combining Analytical Methods with Widget Programming by Ronn Kling
Many examples of writing widget (GUI) programs that allow for interactive analysis in IDL.
Calling C from IDL: Making Sense of the Sometimes Confusing World of C and IDL by Ronn Kling
A great book for those entering the world of dynamically loadable modules (DLMs). This is the most sophisticated way to call C from IDL. Ronn gives many good examples (included on the accompanied disk) of how this works that go beyond the examples found in the External Development Guide.
Power Graphics with IDL: A Beginners Guide to IDL Object Graphics by Ronn Kling
A good introduction and reference to many of the capabilities and properties of object graphics. The key strength of this book are the examples. This is the only book out there that discusses object graphics.

Information design

Beautiful Evidence by Edward Tufte

General programming