FYI if you want to reference another sheet within that other spreadsheet, using the ' when the sheet name has spaces doesn't work, I found out that you need to just leave them out, using the example above:
=importRange("tgR2P4UTz_KT0Lc270Ijb_A","Sheet Name!A1:A10")
instead of
=importRange("tgR2P4UTz_KT0Lc270Ijb_A","'Sheet Name'!A1:A10")