By far the best and most accurate guide to London restaurants. A comprehensive guide very similar in format to Zagat guides with little comments in "" woven into the text, but where it far surpases Zagat is in the cost estimates. Zagat has no defined methodology for pricing - they just ask reviewers how much their meal cost - not taking into account whether the people had tap water only or 2 bottles of wine each. Harden's defines the cost estimates as 3 course dinner with 1/2 bottle house wine, coffee, cover, service, etc. Still only an indication, but in my experience much, much more accurate than Zagat (where I've had cost estimates of £45 come out at £75 per head for a basic meal with little alcohol - that's quite a difference).
I also appreciate the feature which places the restaurants on actual maps. I've used this a lot when people ask me "what's the name of that [italian/indian/french/chinese, etc.] restaurant on [XXX] street."
Great guide to London Restaruants. Recommend it highly.