The following ENVI reference books are available for purchase on Amazon.
Image Analysis, Classfication, and Change Detection in Remote Sensing: With Algorightms for ENVI/IDL, Second Edition – Morton J. Canty
Numerical Recipes 3rd Edition: The Art of Scientific Computing – William H. Press, Saul A. Teukolsky, William T. Vetterling, and Brian P. Flannery
The Visual Display of Quantitative Information, 2nd Edition – Edward R. Tufte
Map projections--a working manual – John Parr Snyder