I have a workbook with multiple worksheets. Each worksheet has a number of
text entries in a column. Some of the entries are repeated on more than one
worksheet. Is there a formula I can construct that will return the number of
unique text entries across the whole workbook instead of just one sheet?

Example:
Sheet 1
A
B
C
D

Sheet 2
B
E
F
G

Sheet 3
D
E
I
J

Unique entries = 9

What formula can I use to generate that '9'? Thanks.