West End Avenue and vicinity was once a stronghold of town houses. Three of the now-rare breed bear distinctive traces of the attentions of recent owners.
Real estate investigation, a big business in a borough like Manhattan with plenty of rent-regulated apartments ripe for exploitation, has picked up in the past year.
The Avery Architectural Library at Columbia University has put part of its collection of real estate brochures online. They date from the 1920s to the 1970s.