I don't think I've bought a strategy since back when Killer Instinct hit the Nintendo 64. I generally disagree with them both in principal and in quality.
However, when I saw the Dark Souls guide that Future Press put out, I had to grab one. It was backordered when I ordered it two weeks ago (and looks like it is again), but my copy showed up this weekend.
I've already finished my NG playthrough, but as with Demon's Souls, I'll likely play Dark Souls through at least NG++ over the next year or so.
The book is a great thick hardcover, and the information is extensive. I really wouldn't urge someone to pick it up for their first play through, since a large part of Dark Souls' enjoyment comes from its mystery. Dark/Demon's Souls are the types of games wherein you can miss something if you're not in the right place at the right time, or you can notably change your gameplay experience if you do/don't kill certain NPCs. Your first playthrough should be trial and error.
However once you've gone through the game once, you may want to make sure you see more of it your second time through, and this book is a great resource for that. It's got wonderful maps and descriptions of areas, including some little tips and tricks you may not have known about.
The most valuable part of the book for me though is that it collects the stats for all of the weapons and armor into neat sections, including their upgraded stats, so that at a glance you can compare weapons and armor (and spells) when deciding how you want to spend your precious souls.
I loved Dark Souls, and it's a pretty huge game. A lot of strategy guides seem to function as glorified instruction manuals, with a lot of junk filler. This one for Dark Souls is a nice publication with a lot of great information and lore about the game. Highly recommended for a Dark Souls fan.