The INDIRECT() function will help you here. Create a table with the
names of the workbooks. Say this table occupies cells K2:K31.

=SUM(INDIRECT("["&OFFSET(K1,ROW(1:30),0)&"]Sheet 1'!A1")

This is an array formula (you need to commit with Shift+Ctrl+Enter). It
also requires that all 30 books are open.

HTH
Kostis Vezerides