The following IDL reference books are available for purchase on Amazon.
Object Oriented Programming with IDL – Ronn Kling
Practical IDL Programming – Liam E. Gumley
An Introduction to Programming with IDL: Interactive Data Language – Kenneth P. Bowman
IDL Programming Techniques – David W. Fanning
IDL Primer – Ronn Kling
Application Development with IDL – Ronn Kling
Power Graphics with IDL A Beginners Guide to IDL Object Graphics – Ronn Kling
Calling C and C++ from IDL: Making Sense of the Sometimes Confusing World of C and IDL – Ronn Kling
Modern IDL A Guide to IDL Programming – Michael Galloy
Wavelets: A Concise Guide – Amir-Homayoon Najmi