It definitely pays to know some specific things for specific aircraft... correct airspeeds, power and flap settings, etc...
But you may find a real-life airliner manual to be a bit overwhelming and not very useful for flight-simming.
There are "distilled" versions for flight sim- Werner Schott made up some excellent ones; just do a search of his name in the Downloads section here at SimV. The files are even made to be printed up into little booklets which are pretty handy.
There are probably many similar manuals out there, but these are the ones I've used, and they are excellent.