References ISAR - Inverse Synthetic Aperture Radar
A couple of books I'd recommend that are similar to the Richard's book

Stimson's "Introduction to Airborne Radar" has a chapter on ISAR.
For a bit more detail you can look at
Wehner's "High Resolution Radar"

If you look at the Polar Format algorithm in SAR (I'd recommed Soumekh's - Synthetic Aperture Radar Signal Processing) it is very similar to the ISAR algorithms after you corrected the target's translational motion (this is not necessarily trivial). Soumekh's book doesn't really address the ideas of motion compensation, target tracking, and range cell migration - these are essential concepts if you are implementing an ISAR algorithm on Airborne data. If you are just trying to get an idea of ISAR processing then you just need a little bit about them.

A good book that covers a wide range of ISAR topics is:"Inverse Synthetic Aperture Radar Imaging: Principles, Algorithms and Applications" by Chen and Martorella.

One piece of advice, if you are reading papers - pay very close attention to the type of transmitted signal and the type of range compression that is used. Often authors just state the signal after "preliminary" processing is given by ... . If you do deramp on receive vs traditional matched filter, the nature of the signal is very different. Similarly, if a stepped frequency waveform is used then the resulting signal processing is different. This is one of the reason's I liked Soumekh's book.

One other book is:Inverse Synthetic Aperture Radar Imaging with MATLAB Algorithms by Caner Ozdemir. This book explains the concepts well, but I was unable to follow the mathematical development and the Matlab code is terrible. You can't easily modify it change the processing parameters.