I have a worksheet with a number of circular references. I have
iteration turned on, and it seems to do what I want. But, if I make
some particular mistakes (eg: entering really unreasonable data), I
get into a situation where most of the cells are #VALUE. The only way
I've found to clear this is to re-load a good copy of the sheet.
'Undo' has no effect.

Are there better ways to recover from this?

More generally: my knowledge of Excel is kind of spotty. I don't have
the time to become expert in Excel, but sometimes I want to use some
particular more advanced feature. The built-in help seems kind of
limited, unless I missed something in the installation. So, I
flounder around.

Is there an on-line "Excel reference manual", or something?

Thanks,
George