We were there in early October 2008. Foolishly, I did not reserve tickets in advance, figuring I would be able to get some on the spot. We had just a day and a half in Granada, and upon arrival I tried to get tickets, but was informed they were sold out. D'oh!
Front desk staff at our hotel (Hotel Anacapri, from The Book) saved my butt, and was able to get us tickets (Kathy, who speaks Iowan, was wonderful, just as the book predicted). But don't count on that - we got very lucky. I saw numerous angry people who just showed up and wanted to buy tickets; they were outta luck, and despite making a big scene (or perhaps because they did), the ticket staff was unsympathetic, and they didn't get tickets that day.
I'm not sure one necessarily needs a guide there, though others might disagree.