The purchase request form at the Yale University Library or a quick email to me are the best ways to get things that we don't already have. Yale also acquires some books in large University Press eBook bundles, and in those cases, we do not actively collect print. However, we do some duplication of eBooks in our print collection when a community member asks for a specific book. For ordinary cases — e.g., when something is just needed for a quick consult — I recommend checking Borrow Direct first.
In Quicksearch, 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 subject tags. Note that, when you click on subject terms on a record, they're nested within the hierarchy. Clicking on Devonian in "Paleobotany > Devonian" won't get you to the same list of books as clicking on Devonian in "Paleontology > Devonian" — if we want to see all books about the Devonian, we would do a subject search for Devonian in the main search box using the drop-down menu, which will match the term wherever it's found.
Below is an example of doing a general subject search in the catalog.
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 (e.g., QK980 C35X 2012), 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.
Here, I'm searching for QE760, which will show me — in alphabetical order — everything with that subject.
We can also click on the Orbis record for any of these to see the subject terms in a new way — in a list so we can see how they're related to proximate terms.
Let's start on the Orbis record for Frozen in Time.
The subject terms show what is near the term Fossils > Antarctica — primarily other fossil locations. This is useful for orienting oneself to how the subject hierarchy works, as knowing how things are organized can improve your future searches.