I used this app constantly last week while in London. I had downloaded a few but found this to be the most useful. Make sure you download the full content including the maps. Most helpful was the city map and whats great is that the GPS works even when you are not connected to wifi or data.
The information about tube stops near sites is helpful and even though the info about tourist sites is limited to wiki info, its still helpful.
Rather than carrying a book or a map, carry your iphone and look less like a tourist.
MatteoV about London Travel Guide Offline