You might be wondering why a English 381 student would consult subject encyclopedias and other tertiary sources. The quick answer is that it can't hurt to make sure you have a good sense of how your research question fits in with other scholarly research that has been done--and the bibliographies at the end of encyclopedia articles are an efficient way to make sure you know who the scholars are who've been working on this topic.
Remember that tertiary sources are intended to fill in gaps in your knowledge or jumpstart your research; they should not be cited as scholarly secondary sources for your project.
The Milton Encyclopedia by 
The Age of Milton: an encyclopedia of major 17th-century British and American authors by 
Literary Research and the British Renaissance and Early Modern Period by Cambridge Companions are a series of authoritative essay collections that synthesize the most important aspects of a topic. Each volume is edited by a leading scholar in the field and offers essays written by experts. Look for Companions on specific authors, genres, themes or movements, and time periods. The companions below are especially relevant for the works of John Milton.
The Cambridge Companion to Paradise Lost by 
The Cambridge Companion to Milton by 
The Cambridge Companion to Writing of the English Revolution by Literature Compass publishes peer-reviewed, state-of-the-field articles from across the discipline. Review articles summarize the current scholarship on a topic and offer an analysis. They can help you identify the scholars working on your topic, recent developments, current debates, gaps in research, and also give you an idea of where the research might be headed next.
More survey articles:
Oxford Encyclopedia of British Literature by 
The Oxford Encyclopedia of the Reformation by 
The Oxford Encyclopedia of the Bible and the Arts by 
