This comes from The Rings of Saturn by W.G. Sebald:
All I know is that I stood spellbound in his high-ceilinged studio room, with its north-facing windows in front of the heavy mahogany bureau at which Michael said he no longer worked because the room was so cold, even in midsummer; and that, while we talked of the difficulty of heating old houses, a strange feeling came upon me, as if it were not he who had abandoned that place of work but I, as if the spectacles cases, letters and writing materials that had evidently lain untouched for months in the soft north light had once been my spectacle cases, my letters and my writing materials.
I have no idea why a semi-colon is used. We usually use it when a dot is appropriate, but in this case it's not, because it's connected to the very first statement, and thus is not an independent clause.