William Poundstone's "Big Secrets." It only has one chapter on numbers stations, but it's damn good and the chapters on other topics are damn good, too.
There is an android application to be notified when it is transmitting and that allows you to listen to them via WEBSDR without searching them. https://play.google.com/store/apps/details?id=com.tortillum.slurmnumbersstations
Thank you for this! Downloaded! Here is the US link to it:
A search on Amazon shows several copies going for as low as $15 dollars.