Highline College has a LARGE collection of Reference books.
The best Reference books for background information are "subject encyclopedias." Like Wikipedia, they provide comprehensive overviews of topics. Unlike Wikipedia, each article in a published Reference book is usually written, and signed, by an expert scholar, so it can be cited in your research project.
A search of Gale eBooks helps you find electronic reference books published by Gale. Gale is very easy to use especially because you can look through its entire collection with a single search.
Here's an example of a search using "freedom of religion" as the topic. Once you get your results, notice that you can print, email, or download the article. In some cases, the database will even read the article aloud to you. And, as a bonus, it has a "Citation Tools" feature which will create a bibliographic citation for you. What could be better?