Anyway, my theory is that the diary isn't hers. It either belongs to a future companion, or Amy Pond, or his actual future wife, etc. I imagine she might have some relationship with the Time Agency of the 51st Century. Is he meeting her all within the timeframe of her linear life? Or is she hopping time, too? What year was Silence in the Library set in? The same century as this one? So, really it's more The Doctor swinging in and out of her lifetime, than her going around time finding him. OR, she's a Time Agent, and she's playing a con. She will probably find herself turning good by the end, due to The Doctor's influence, and sacrificing herself (with him none the wiser of her identity) in the Library. Thus, her more earnest demeanor in SotL, than here. OR, maybe she's who Amy Pond grows up to be?
Or, I could be wrong?