Free
New edition of a self-published book I vouch for.
London: A Visitor's Guide: 2016 New edition - updated Dec 2015 by Craig Cross
http://www.amazon.com/dp/B00JUXW2E0/
http://www.amazon.co.uk/dp/B00JUXW2E0/
http://www.amazon.ca/dp/B00JUXW2E0/
Craig Cross is the guy behind the London travel blog at http://www.londondrum.com/blog/.
He has spent the last few years as a full-time tourist, visiting nearly 200 different landmarks, attractions and hotels in London, so he can compile them into a huge tourist guide which collects together all of his experiences, observations, hints and tips into a single 1,000 page volume.
Note that if you bought the previous edition you need to contact Amazon - no automatic update will be sent yet Amazon stop you buying it again.