Complex sentence: While Wallis was researching the book, the Guardian's Office, Scientology's intelligence agency, assigned an individual as an undercover agent who pretended to be a student at Stirling University, where he was teaching.
Split into simpler sentences: While Wallis was researching the book, the Guardian's Office, Scientology's intelligence agency, investigated him. They assigned an individual as an undercover agent who pretended to be a student at Stirling University, where he was teaching.