Primary and Secondary Resources should be your GO-TO when constructing your argument and writing your research paper
Primary Resources
Original documents, letters, art work, etc.
Things written or created by someone whose work and/or ideas you are exploring.
For example: Tillich, Paul. Systematic Theology. Chicago: University of Chicago Press, 1951.
This seminal work might be one of your primary sources if you were researching his thought in relation to creation and nature.
Secondary Resources
Books/articles written by someone who is commenting on your primary source or artifact.
Books/articles whose focus is in the same or similar subject area.
For example: Haffner, Paul. Towards a Theology of the Environment. Leominster, England: Gracewing, 2008.
While it may not touch on Tillich specifically, this work may provide useful insights to further your conversation with Tillich's ideas.