When it comes to the topography of early 19th century London, this tattered tome is my bible. Based on a survey by Richard Horwood from 1792-9, the map was updated by William Faden in 1813, which makes it perfect for the Sebastian St. Cyr series. It is beyond wonderful; not only does it show all the streets, lanes, courts and alleys, but it even shows individual buildings with street numbers! Unfortunately for those of us with aging eyes, Faden's originally larger map segments have been reduced for publication to pages that only are about 8" x 12", so these days I find myself reaching for a magnifying glass a lot.
When I first started
What Angels Fear, I had yet to discover the London Topographical Society and had to make do with an 18th century map of London and the Greenwood map of 1827, which is on line
here (that's a screenshot from the Greenwood map, above).
Although London in 1827 was amazingly different from London of 1811-13, I still use it sometimes because you can really zoom in on a location and print it out. (Once upon a time I had the entire map printed out and mounted on giant sections of foam board, but then this little thing called Katrina flooded my office, and that was the end of that.)
Also online is John Rocque's 1746 map of London, which is
here. It's also available in book form from the same people who print the 1813 Regency map.
To give you an idea of the kind of detail these maps provide, here is Hyde Park corner from 1746, showing the turnpike, the Tiburn gallows, and the nearby spot where, the map tells us, "soldiers are shot."
I love these maps and spend hours pouring over them just for fun. Some of us are easily amused.
As a reader I appreciate that you use maps in your research. It always bothers me when characters in other author's books are seen walking down streets that didn't exist at the time.