Book Value is mostly meaningless.
Take a look at Amazon - selling for $354, book value is $20.
How about IBM - selling for $182, book value is $18.
And what 'downtrend' are you talking about. Oh I see. You mean the last week and a half. So you ignore the last 1, 2, 3, 4, 5 years of strong uptrend with an occasional dip like this one.?? Hell, in mid-2011, there was a 33% dip from about $97 to below $70. Since then MIDD has gone from $70 to today's $219.
This dip is just a blip in the long term uptrend of MIDD!
So yeah. Go ahead and sell.