There are two good options for finding an e-book.
There may be some overlap between the two.
This video shows how to use the ADVANCED SEARCH option within the catalog to search your keyword and limit to online materials.
This video shows how to search by SUBJECT HEADING and then limit your search to online materials. It has a different "limit" screen than the advanced search screen in the video above.
In each of these platforms, you can search by author, title, subject heading, or keyword. I recommend searching in the EBSCO platform first. I find it the most user-friendly and with the least restrictions.