With book 2 and book 3 in columns A and B (let's say from A1:B8), put this array formula in C2 (you must leave C1 blank above it) and confirm it with CTRL+SHIFT+ENTER:
=IF(ISNA(INDEX($A$1:$A$8,MATCH(0,COUNTIF(C$1:C1,$A$1:$A$8),0))),INDEX($B$1:$B$8, MATCH(0,COUNTIF(C$1:C1,$B$1:$B$8),0)),INDEX($A$1:$A$8, MATCH(0,COUNTIF(C$1:C1, $A$1:$A$8),0)))