Buy the book, there should be no distinguishable difference in how a Wizard casts a spell, and a Sorcerer casts a spell. Its the same verbal and somatic components (and in PNP, the same material components).
The only difference in casting would normally be applying metamagics in a pen and paper environment... in which a sorcerer then be using a full-round action, instead of a standard action (thus, it would be more noticable that they have to add more to the casting incantations). In NWN nights, though, there sorcerers don't suffer this slow-down, either.
A spellbook has nothing to do with casting, it is only for 'preparing the spell'. If a wizard prepared, and then burns his book, he can still cast what he has prepared.
So, they really shouldn't be able to tell the diffence (they'd only be able to do that when the character is 'preparing' his spells. And then, i'd think it'd be very tough for a sorcerer to 'fake it', as they have to do their preperation properly, not be fake-reading a book.
But, as EO mentioned, there is very little mechanically that can be done to alienate the sorcerer class in a place that should do so.. so I see this as a fitting incorperation.. even if its not completely 'proper' for how it should be handled it acheives the same effect.