I have posted this link before: https://www.loc.gov/collections/sanborn-maps/?fa=location:louisiana%7Clocation:ouachita+parish which takes you to the Library of Congress Digital collection of Ouachita Parish Sanborn maps. When I posted, the collection only went up to 1898. Now, more has been added online! They have maps of Monroe and West Monroe all the way up to 1926! These are great because they tell you what the buildings were made of and sometimes what businesses were there! Unfortunately, these are only for buildings inside the city limits at the time so if your ancestor lived outside the town, you are out of luck.