There are two types of books to mention, those on where to find the art, covered on this page, and those on technical metters, dealt with elsewhere.
London’s Statues and Monuments (Shire, 2012) was the first I bought and proved useful in getting started.
Kershman's London's Monuments (Metro, 2013) is now my book of choice as it is better organised.
London's Cemeteries (Metro, 2011) is another Metro book, equally well done and sometimes useful but not as relevant to the mission.
Lynn Pearson's Public Art since 1950 (Shire, 2008, acquired April 2018 for pence) is 64 great pages packed with pieces I've never heard of before, throughout the UK. Many will be added to the new targets list.