Bruce Alexander has a series of novels about Sir John Fielding who started the Bow Street Runners, unfortunately only the last couple are ebooks.. Here is one of 'em
http://www.ebooks.com/ebooks/book_di...asp?IID=356754 You'll have to find the earlier ones in print.
Some of the best David Liss novels are set in the 1700s London and he's the better writer. His first of three is called "A Conspiracy of Paper" is absolutely great, but not ebook'd, however the two followups with the same main character are available. His "The Coffee Trader" is good stuff too.