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In English, for native speakers to correct:
"She had been in that bookshop twice before, and she had been perusing dust jackets all morning when she suddenly realized that she was being watched by a mysterious man".
I think the difference you are asking about is the progressive/continuous aspects (imperfective aspects), often used to describe actions continuously or periodically done in the past that were interrupted by another action. With the past perfect the action is completely done in the time frame of the sentence and we only keep its notion and enduring effects. Past perfect progressive adds the notion of an action "in progress" that is completely done in that time frame, because it ended before or it is interrupted by another action.
So far it's very similar to our Spanish tenses, but Spanish imperfect may move some boundaries:
She had been in that bookshop ---> ella había estado en esa librería
The ceremony began when everybody had arrived to the church --> La ceremonia comenzó cuando todos hubieron llegado a la iglesia -bookish- (La ceremonia comenzó cuando ya todos habían llegado a la iglesia -non bookish-)
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