This website, from the Carnegie Mellon University , includes, not only books on-line, but art, music, and videos. There is also a separate Shakespeare collection. I does not assign classic Greek and Roman works, other courses do. This MIT site offers e-text of many such ancient Greek and Roman works. Even if you briefly refer to one source in an essay, you must cite your source so that you are not accused of plagiarism.
Different disciplines require different forms of documentation.
The following might help. Several pages explaining, in detail, how to write a lab report. The table of contents lists links to the specific steps and advice any writer should follow. This is an online course, with 22 lessons, on scientific and technical writing . When reading literary criticism or writing about literature, you will come across literary terms which are necessary to understand.