I just received a copy of IDL Primer by Ronn Kling. (Full disclousre: I received a free copy because Ronn used my Periodic Table of IDL Operators on the back cover.) It’s a pocket sized “quick reference” that provides a quick introduction to the main functional areas of IDL: the IDL Environment (DE is covered, get Kling’s Navigating the Workbench for coverage of the Workbench), syntax, array operations, program flow, 2D and 3D graphics, image processing, object graphics, file I/O, mapping, and animation. I have found it to be a quite handy reference, particularly the list of “Important Routines by Function” at the back (similar to the Functional List of IDL Routines in the online help).

Periodic Table of Operators on backcover of IDL Primer