If you are going to place the current month information in A1 for each new sheet, so that we can use that to reference previous months' sheet, then change the entry in A1 to a true date, like Feb 1, 2010 and format as mmm yyyy.

Then assuming you want the last number entered in column B... try:

=LOOKUP(9.999999E+307,INDIRECT("'"&TEXT(EOMONTH($A$1,-1),"mmmyyyy")&"'!B:B"))