There are some key differences between Books and Dissertations:
"Books" refers to a text that has been reproduced in a mass quantity, and is typically available for purchase in a variety of places. The writing in books tends to be geared toward specific audiences, which range from college professors all the way down to elementary-age students. Some books originally began life as dissertations, but in most cases when a dissertation is published as a book, it is edited in a variety of ways based on its new target audience. Books are usually produced after a certain subject has been researched quite a bit -- the more recent a research topic, the fewer books will be published on it. (So for instance, there will not yet be any books published on COVID-19 in the Pacific; there will be some books published on sea-level rise in the Pacific; and there will be many books published on the revival of traditional navigation techniques in the Pacific.)
This video will give you some tips on how to find online books, using the resources below: VIDEO 2: Finding Books
"Dissertations (and theses)" are written by scholars for scholars, as part of their progress toward either a PhD or a Masters degree. They are not generally produced for large-scale distribution (though they are often revised and published as books). The document produced by a PhD candidate is typically referred to as a Dissertation, while the document produced by a Masters candidate is usually called a Thesis. Dissertations and theses are written for faculty committees that review them to ensure they meet certain academic requirements. They include things like chapters on the methodologies that the researchers used. The writing can be highly academic. Dissertations and theses can often be useful because they include bibliographies of all the works the researchers studied while writing their dissertation -- so even if you don't read the dissertation itself, the bibliography can help you find other resources on a given topic.
This video will help you find dissertations: VIDEO 3: Finding Dissertations (when you can't find books
OneSearch and HathiTrust are both places where you can find electronic books; when you use OneSearch, you are also searching for books within HathiTrust, but there are reasons to search in both places:
OneSearch is a "search aggregator," which means that it searches for a wide variety of resources (books, films, journal articles, audio recordings) in a wide variety of places (within Hamilton library, in various electronic databases, on the open internet and elsewhere). So the advantage of OneSearch over HathiTrust is that it searches a much broader "universe" of information than HathiTrust, and will give you access to electronic books that are not available through HathiTrust.
HathiTrust is also a type of search aggregator, but it is more limited than OneSearch, in that it only searches for electronic books and only searches for those books that are freely available from a select group of libraries that are members of HathiTrust -- so there are electronic books that you will find in OneSearch that you will not find in HathiTrust, because OneSearch also searches for books that the library pays for, and which aren't feely available through HathiTrust. The advantage to searching directly in HathiTrust instead of in OneSearch is that, for the books that are in HathiTrust, HathiTrust can be used to search through the entire text of all the books it holds for the keywords you are using (OneSearch does not search the full-text of electronic books, only the "metadata" -- the title, the author, the publisher and so forth).