gingertips,
One function to use is

=TEXT(A2, "mmm") or TEXT(A1, "mmm-yy")

This can create an additional column (say C:C), which will extract the
month portion. Use the second version if you also need to distinguish
between years. This function is for next to the data and will be used
by the summation functions.

For summation, assuming your months are in A:A use

=SUMIF(Sheet1!C:C, A2, Sheet1!B:B)

If you have them in different workbooks use clicking for the arguments
while editing the formula, this will properly enter the full
workbook/worksheet name in lieu of Sheet1 in the suggested formula.

HTH
Kostis Vezerides