_ASSUMPTIONS:
1) You have 5 lookup tables
2) Each table is located in a different sheet
3) On each sheet, A1:B100 contains your lookup table
_FORMULA:
1) If Column B of your lookup tables contains numerical values...
=SUMPRODUCT(SUMIF(INDIRECT("'"&A1:A5&"'!A1:A100"),B1,INDIRECT("'"&A1:A5&"'!B1:B100")))
2) If Column B of your lookup tables contains text (or numerical
values...
=VLOOKUP(B1,INDIRECT("'"&INDEX(A1:A5,MATCH(TRUE,COUNTIF(INDIRECT("'"&A1:A5&"'!A1:A100"),B1)>0,0))&"'!A1:B100"),2,0)
...confirmed with CONTROL+SHIFT+ENTER, not just ENTER.
For both formulas, A1:A5 contains a list of sheet names, and B
contains the lookup value.
Hope this helps!