I feel like the sad goodbye to Amy and Rory at the end of The Angels Take Manhattan in season 7 was unnecessary. I mean couldn't the Doctor have used River's vortex manipulator to go back to when the Book was published then used the "about the author" description to find their address or even go back about a hundred years and wait in New York and still use the book for their address? I mean he could have saved them if he really wanted to right?
I believe the Doctor cannot revisit that part of his timeline, he says that the world would most likely implode (or something along those lines) if he did. Also, once the Ponds were once again sent back in time to New York it is quite likely that something around their stay there became a fixed point in time.
And yes, I think the possible duplicate linked in the comment above may answer your question better than I can at the moment.
No: going back to save them would have caused a paradox and ripped the world apart.
From the episode's transcript:
DOCTOR: Back where we started. You collapsed the timeline. The paradox worked. We all pinged back where we belong.
RORY: What, in a graveyard?
AMY: This happened the last time. Why always here?
DOCTOR: Does it matter? We got lucky. We could've blown New York off the planet. I can't ever take the Tardis back there. The timelines are too scrambled. I could have lost you both. Don't ever do that again.
RORY: There's a gravestone here for someone with the same name as me.
(Rory vanishes. There was an Angel behind him.)
DOCTOR: I'm sorry. Amelia, I'm so, so sorry.
AMY: No. No, we can just go and get him in the Tardis. One more paradox.
DOCTOR: Would rip New York apart.
DOCTOR: You are creating fixed time. I will never be able to see you again.
AMY: Raggedy man, goodbye!
(Amy turns her back on the Angel, and vanishes. Rory's gravestone gains more words - And His Loving Wife Amelia Williams aged 87.)
Out of universe, of course, it was time for Karen Gillan and Arthur Darvill to leave the show, and this was a fancy and poignant way of separating them from the Doctor forever.