We've discussed the Michelin guide at length. What are the alternatives?
The one I always recommend and keep around to lend to visitors is Patricia Unterman's Food Lover's Guide. With rare exceptions (particularly regarding spicy food) I find her judgment trustworthy, and I like how the book covers not only restaurants but bakeries, markets, butchers, and so on.
The Slow Food Guide seems redundant to me, pretty much covers the same ground. (Full disclosure, I was one of dozens of unpaid writers who contributed capsule reviews.)