It's a lot of reading, but the EMP commission report seems to be the most thorough review of the possible impacts of EMP.
I read "One Second After" which paints the end-of-the-world scenario that the OP posits, and then I read the EMP Commission report. My impression was that the actual impact would be awful, but not as bad as the book proposes. The premise of "One Second After" is that almost everything is totally destroyed... all electronics, power, telecommunications, and modern transportation. In reality, only a subset would be affected. For example, from the EMP Commission report:
an EMP attack would disrupt or damage a functionally significant fraction of the electronic
circuits in the Nation’s civilian telecommunications systems in the geographic region exposed to EMP.
Note that it didn't say *all* telecommunications would be damaged. In fact, it later says that there will still be enough surviving infrastructure to overload the circuits from people making calls. Even just a few operational phone lines would go a long way to facilitating emergency response across communities.
However, the report rightly notes that all our infrastructure is interrelated and damage in one infrastructure area can impact others. Notably:
The Commission has concluded that the electrical system within the NERC region so disrupted
will collapse with near certainty. [...] This loss is very large geographically and restoration is very likely to be beyond short-term emergency
backup generators and batteries.
Which means even the parts of the telecommunications infrastructure that survive would be without power to keep it going.
EMP may not send us back to the dark ages, but it is a very serious threat. Also keep in mind that the report was written in 2008, and our dependence on electronics has only increased since then.