[Please note: This review is pretty much the same as my Lonely Planet Paris Encounter guide, because the two books have the same format.]
This guidebook worked out well for me because there is so much to see/do in London besides the major monuments. The author (introduced at the beginning of the book) is a 20- to 30-something year old who suggested off-the-beaten paths that someone like me would enjoy (cabarets, record shops, etc.)
But if you want detailed descriptions about the site you're looking at, this is not the book for you. A few things about this book were not very helpful: 1) you'd be better off buying a laminate map than the fold-out included with this one; 2) the interior maps number each site, but the descriptions don't show the number...this makes it very hard to reference where you're going; 3)the features of what the locals "think" about London is interesting, but is (of course) one-sided and not very necessary; 4) author doesn't tell you all the must-see places (pretty biased to what she enjoys about the city).