YTA, but hear me out. You have the same problem I do: You're a pessimistic problem solver. When you hear an idea, your mind immediately goes to the negative outcomes and the obstacles in the way. This can be a great trait to have, but interpersonally, it makes us giant downers. Especially when paired with optimists like your girlfriend.

Before diving into problem solving mode, ask yourself: Is she likely to follow through on this, or is this just daydreaming right now? If so, is my input timely or am I solving problems that don't exist yet? Talking about how she'll need to hire an editor, for example, isn't necessary in response to an offhand comment about writing a book that will almost certainly never come to fruition.

If you do feel like your input is warranted, frame it positively. Instead of talking about how living on the outskirts will be too expensive and onerous, focus on why living in the city will be awesome. Instead of saying you don't want to proofread her book, say that while you'd be happy to share your thoughts, a great editor can really transform a book.

And finally, take my advice with a grain of salt, because I suck at following through on it myself.