In the new QuickSearch tool, you can search for general topics and narrow down to specific subjects using the facets to the left of your search results. This can limit resources to specific date ranges, languages, locations, online/offline, dissertations, et cetera.

On a record, you can also click on the subject terms assigned to a book relevant to your research. This will help you with digital serendipity by calling up books from across YUL that have those same subjects. When you find a topic you think you will be searching frequently, write it down — you may decide to run the search again!

In the old search interface, Orbis, you have a really interesting option that might be helpful to you. Library of Congress call numbers, which appear in either the catalog record or on the print spine, can be searched. These call numbers are based on book topics, and you can locate other materials related to the book of interest to you with this search feature. In the image below, I've typed in the call number I want to search, and I am selecting Call Number (LC). 

Here, I'm searching for P299, which will show me — in alphabetical order — everything with that subject. Note that I have stuck with the first row of the call number. The second level on the spine of a book is usually a code for the author or volume editor, so it's not necessary.