Background pony is probably my favourite one, it's so good that I consider it the best book I've ever read. The story is very interesting. It's perfectly implemented in the show's events and goes with canon plot side-to side without breaking it. The plot leads to an outstanding ending that answers every question, gives answers to every single mystery listed in the book (and there are a lot of them) and ends this fic in a way that nothing could be added. It's a perfect, finished and very sad story. I'd recommend everyone to read it. I don't even want to start reading Past Sins because I feel that nothing could beat Background pony for me, but I'll give it a shot soon.
You probably should, I'm sure you'll like it as mch as I did.