Icelandic 'sem'/Swedish 'som' etymology?

Hi!

What is 'sem' in Icelandic derived from?  Or Swedish 'som' or Faroese
'sum' (i.e., the modern North Germanic relative clause particle)?
I could not find a convincing etymology online and unfortunately don't
seem to have the appropriate books.

And going back a few years: what is the etymology of 'er', also
relative pronoun, but in Old Norse?

Bye,
  Henrik

PS: I'd also be interested in book recommendations for Norse/Icelandic
    etymological dictionaries.

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