A good primer, but more like SIGINT 101 CIRCA 1970. No mention of China, Iran, North Korea, or the use of SIGINT against terrorist or insurgent networks.
As interesting as SIGINT against terrorist networks sounds, I assume most of this is classified -- since the cells are already known to operate smaller, and likely more difficult-to-intercept networks than nations, they'd probably adapt fairly quickly if it were revealed how they're being eavesdropped on.
I would imagine that since groups like ISIS aren't running their own cell networks, it would be very easy for government groups to locate anyone using those networks. Satellite phones can be just as easy to find too. The only way to really hide is just blend in with the normal cell noise of a city, use encrypted messaging, and hope no one is connecting the metadata dots to find you.
Mexican drug cartels have been found to be running their own cell networks, so the idea that ISIS or any other well-funded group could do the same is not outside the realm of possibility. Of course, from a SIGINT perspective, cell towers are pretty hard to hide.
> China, Iran, North Korea
For states, everything stated in the OP still applies.
"Your emails may not be as private as you think". When has email ever been private? Even if you're using PGP there is still the issue of metadata leaks and people who don't understand PGP and reply with the unencrypted text in the reply (botching your attempts at secure comms). Not to mention people storing their whole life in a single email account and making it easier to build up a dossier of the person over time.