I followed advice offered via the D.G. to ensure that: (1) the Excel > Tools
> Options > General 'At start-up open with' box is unchecked and (2) all
'XLSTART' folders are empty; but these actions did not cure the problem.

I then removed all Add-ins to Excel and found that an Add-in program called
'Excelman' was causing the problem.

Excelman was a useful and commonly available free download, but nothing
comes up when I search for an up-dated version in the hope that it may have
cured its bug causing Book 1 opening. Does anyone know what happened to it?