In-text reference a study guide without an author

Here are a couple of common ways to do it:

1. Using the full title: If the study guide is titled "Introduction to Quantum Mechanics Study Guide," you would cite it as: ("Introduction to Quantum Mechanics Study Guide," Year)

2. Using a shortened title: If the full title is very long, you can use a shortened version, usually the first few significant words, enclosed in quotation marks: ("Quantum Mechanics Study Guide," Year)

Example:
The principles of quantum mechanics can be complex, requiring dedicated study ("Introduction to Quantum Mechanics Study Guide," 2021).

If the study guide has no author and no publication year, you would use "n.d." for "no date":
("Study Guide," n.d.)