As I recall, Shelkunoff did cover the near-field region. In round numbers, the practical distinction between DC and AC was the ratio of shield thickness divided by skin depth at the frequency of interest.
If the shield was multiple skin depths in thickness, the concept of inside and outside currents is realized, and the coupling between inside and outside becomes exponentially small as the ratio grows.
Below that frequency, there is no distinction between inside and outside, and the transfer impedance of the shield is basically the DC resistivity of the shield.
I first ran into this while deigning coax systems to distribute time reference signals, where mains power frequency ground-loop currents could couple voltages efficiently onto the center conductor of Heliax cables with 100 dB/meter shielding effectiveness at 10 MH to 100 MHz. Even coax using copper water pipe as the shield was going to have trouble with those ground loops.
The solution was to make the reference-signal receivers transformer coupled. The transformer was like five turns wound as a tororid on a one- or two-hole ferrite bead. There was a RF coupling capacitor between coax center conductor and the input of the RF transformer, which transformer had essentially no coupling at power frequencies.
The coupling capacitor reduced the power-frequency current through the transformer enough to prevent saturation of the transformer core, and the transformer eliminated any power-frequency component that did get by the capacitor from making it though the transformer.
Because skin depth depends on both magnetic permeability and electric perceptivity, ferromagnetic metals like mu metal can perform EM shielding at fairly low frequencies. Actually, ordinary mild steel ain't bad either, and is a whole lot less fussy than mu metal.
Joe Gwinn