Variable remains empty in VBA Do while iteration

I am having trouble w/ my macro not capturing the object/variable "book". It should pull the string from the cell, then test if its equal to the string. My code only captures the first string, and then becomes empty after the first iteration.

     Dim i%, j%
        i = 37
        j = 1
        Dim TRBfound As Boolean
        Do While TRBfound = False
            book = Cells(i, 1).Value2
            If Left(book, 3) = "TRB" Then
                TRBfound = True

            Else

                Sheets("Sheet2").Select
                Cells(i, 1).Select
                Selection.Copy
                Sheets("Sheet18").Select
                Cells(j, 1).Select

                ActiveSheet.Paste
                i = i + 1
                j = j + 1
            End If
        Loop

        End If

Data: beginning in A37, which is also merged with other cells next to it.

AOU - AOU
DSU - DSU
TRBK - Treasury Book
LLE - US Single Stock
SDC - SDC
SZK - AMRS DELTA 1 RIS
TRBK - Treasury Book
TRBK - Treasury Book
TRBK - Treasury Book
TRBK - Treasury Book
TRBK - Treasury Book
TRBK - Treasury Book

Variable remaining empty in VBA Do while interation