Even though Google attempts to screen out "content farms" your search results, particular for English-related topics, vary in quality.

Here are results of a Google search for Poe AND symbolism AND Ligeia. These include a free, full text e-book but they also include Spark Notes (Think Cliff Notes) and an article in Wikipedia, a tertiary source. Clearly you can do better with GALILEO or GIL-Find if you are looking for full text scholarly articles or for books.

If you really want to stick with Google, Google Scholar is a better choice for research, even though it is not a hundred percent full text.