The first murder in What Darkness Brings takes place in an ancient lane just off the Minories in St. Botolph-Aldgete. So where exactly is that?
St. Botolph-Aldgate was a long, thin, irregularly shaped parish stretching from the Thames and the Tower of London, in the south, up to Houndsditch in the north. In Sebastian's day, it was peculiar in that it straddled the boundary between the City of London and Middlesex, which created all sorts of administrative issues we don't need to worry about. A densely populated, poverty-stricken area of London, it was home to a few rich and middle class people and a lot of laboring poor involved mainly in food provisioning, warehousing, craftwork, and a variety of trades connected with the river. Whitechapel Road and Ratcliff Highway ran through it; this is also the site of Rosemary Lane, where Sebastian buys the old clothes he uses in his disguises. Here's a rough map that will hopefully help orientate you; it's hand drawn because I don't want to fall afoul of anyone's copyright. If you click on it, a larger image will come up which will be easier to see.
The Minories is the major street running from the Tower up to Whitechapel. Its name comes from the Abbey of the Minoress of St. Mary of the Order of St. Clare, which was dissolved by good old Henry VIII back in the 16th century. The chapel of the former abbey became the parish church of Holy Trinity, while some of the old abbey buildings were used as an armory for the Tower. The area escaped the Great Fire of London, but most of the abbey's surviving buildings had to be torn down after a 1797 fire hit the area between Church and Haydon Streets. Also late in the 18th century, the East India Company started tearing down great swaths of old buildings in this area to make room for their vast warehouses.
"Fountain Lane" is my own invention, although the name is a hat tip to an ancient inn in the area called "the Old Fountain," demolished in 1793. The church of the Holy Trinity was eventually decommissioned, then destroyed in World War II; the street known as the Minories is still there, but Sebastian would never recognize it today. Pay attention to the area to the east of the Tower called St. Katherine's; this will be the site of the murder that opens book #9, Why Kings Confess.