Start with a keyword search in the catalog. Once you have located a relevant item, you can look at the Library of Congress Subject Headings within the full record tab and follow those links to find related books.
You can also refine your results by using the limiters on the left side of the screen. If you are looking for online sources only, narrow down your search by clicking the box next to the "available online" option.
To search for books by an author, use an Author Search. To search for books about an author, use a Subject Search.
To search for a Topic, use logical keywords to identify several relevant records, click on the title and look at the full record to determine if there are Subject Headings that might expand your search. Scroll half way down the page and you will find a Summary of the item, the Table of Contents, and the Subject Headings that are linked.