Hi Jason,
Summing two ranges from two sheets in same book:
=SUM(Sheet2!C1:C6,Sheet1!A1:A6)
Or to sum lots of pages =SUM(Sheet1:Sheet23!A1:B2)
Summing two ranges on two sheets on different books:
=SUM([Book1]Sheet1!$A$1:$A$6,[Book3]Sheet2!
$C$1:$C$6)
Best wishes
Bernard