http://www.princeton.edu/sgs/publications/articles/effects/effects-11.pdf
Glasstone’s chapter on EMP. The kind that zaps a huge portion of land under it is the High Altitude EMP or HEMP pulse, from a very high detonation. 11.13 is the start of that section.
The detonations lower down produce EMP (any nuke does) but not as intense and mostly local, where other effects like blast and neutrons give you a lot more to worry about than your iPod not booting.
Geeky and somewhat pedantic Glasstone’s book is the best unclessified tome on nukes available.
Thanks