I spent a week exploring the rabbit hole of Numbers Stations recently. Of course the ultimate and definitive sound recording on the subject is
The Conet Project. I've had the official download of the box set for years but only recently gave it the attention it deserves.
If you haven't already, be sure to pick up the updated Fifteenth Anniversary edition from 2013 which introduced "1111 CD5" - a fifth bonus CD comprising 26 tracks of polytonal noise stations. And make sure your copy includes the 79-page PDF of the booklet - it has lots of useful information.
I've found a few really cool independently published books on numbers stations, but they were small-run publications and cost around $240 USD for a copy. Original physical copies of
The Conet Project are similarly rare.
After watching the BBC documentary
Tracking the Lincolnshire Poacher and a number of other short films on the subject, I tracked down newsletters by ENIGMA2000 for the latest reports on numbers stations, and then I researched SDRs and discovered
websdr.org - a wonderful software defined radio service to tune in to the frequencies mentioned in the newsletters live.
I was excited to learn that, as recently as January 20, 2023, hackers have been hijacking long-standing stations like The Buzzer and hiding images in the spectrogram. I'm glad that even after all these years numbers stations are still topical.